Switch header convention from #include "SDL.h" to #include <SDL3/SDLh>

I ran this script in the include directory:
```sh
sed -i '' -e 's,#include "\(SDL.*\)",#include <SDL3/\1>,' *.h
```

I ran this script in the src directory:
```sh
for i in ../include/SDL3/SDL*.h
do hdr=$(basename $i)
   if [ x"$(echo $hdr | egrep 'SDL_main|SDL_name|SDL_test|SDL_syswm|SDL_opengl|SDL_egl|SDL_vulkan')" != x ]; then
        find . -type f -exec sed -i '' -e 's,#include "\('$hdr'\)",#include <SDL3/\1>,' {} \;
    else
        find . -type f -exec sed -i '' -e '/#include "'$hdr'"/d' {} \;
    fi
done
```

Fixes https://github.com/libsdl-org/SDL/issues/6575

include/SDL3/SDL_timer.h

@@ -0,0 +1,222 @@
+/*
+  Simple DirectMedia Layer
+  Copyright (C) 1997-2022 Sam Lantinga <slouken@libsdl.org>
+
+  This software is provided 'as-is', without any express or implied
+  warranty.  In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+
+#ifndef SDL_timer_h_
+#define SDL_timer_h_
+
+/**
+ *  \file SDL_timer.h
+ *
+ *  Header for the SDL time management routines.
+ */
+
+#include <SDL3/SDL_stdinc.h>
+#include <SDL3/SDL_error.h>
+
+#include <SDL3/begin_code.h>
+/* Set up for C function definitions, even when using C++ */
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * Get the number of milliseconds since SDL library initialization.
+ *
+ * This value wraps if the program runs for more than ~49 days.
+ *
+ * This function is not recommended as of SDL 2.0.18; use SDL_GetTicks64()
+ * instead, where the value doesn't wrap every ~49 days. There are places in
+ * SDL where we provide a 32-bit timestamp that can not change without
+ * breaking binary compatibility, though, so this function isn't officially
+ * deprecated.
+ *
+ * \returns an unsigned 32-bit value representing the number of milliseconds
+ *          since the SDL library initialized.
+ *
+ * \since This function is available since SDL 3.0.0.
+ *
+ * \sa SDL_TICKS_PASSED
+ */
+extern DECLSPEC Uint32 SDLCALL SDL_GetTicks(void);
+
+/**
+ * Get the number of milliseconds since SDL library initialization.
+ *
+ * Note that you should not use the SDL_TICKS_PASSED macro with values
+ * returned by this function, as that macro does clever math to compensate for
+ * the 32-bit overflow every ~49 days that SDL_GetTicks() suffers from. 64-bit
+ * values from this function can be safely compared directly.
+ *
+ * For example, if you want to wait 100 ms, you could do this:
+ *
+ * ```c
+ * const Uint64 timeout = SDL_GetTicks64() + 100;
+ * while (SDL_GetTicks64() < timeout) {
+ *     // ... do work until timeout has elapsed
+ * }
+ * ```
+ *
+ * \returns an unsigned 64-bit value representing the number of milliseconds
+ *          since the SDL library initialized.
+ *
+ * \since This function is available since SDL 3.0.0.
+ */
+extern DECLSPEC Uint64 SDLCALL SDL_GetTicks64(void);
+
+/**
+ * Compare 32-bit SDL ticks values, and return true if `A` has passed `B`.
+ *
+ * This should be used with results from SDL_GetTicks(), as this macro
+ * attempts to deal with the 32-bit counter wrapping back to zero every ~49
+ * days, but should _not_ be used with SDL_GetTicks64(), which does not have
+ * that problem.
+ *
+ * For example, with SDL_GetTicks(), if you want to wait 100 ms, you could
+ * do this:
+ *
+ * ```c
+ * const Uint32 timeout = SDL_GetTicks() + 100;
+ * while (!SDL_TICKS_PASSED(SDL_GetTicks(), timeout)) {
+ *     // ... do work until timeout has elapsed
+ * }
+ * ```
+ *
+ * Note that this does not handle tick differences greater
+ * than 2^31 so take care when using the above kind of code
+ * with large timeout delays (tens of days).
+ */
+#define SDL_TICKS_PASSED(A, B)  ((Sint32)((B) - (A)) <= 0)
+
+/**
+ * Get the current value of the high resolution counter.
+ *
+ * This function is typically used for profiling.
+ *
+ * The counter values are only meaningful relative to each other. Differences
+ * between values can be converted to times by using
+ * SDL_GetPerformanceFrequency().
+ *
+ * \returns the current counter value.
+ *
+ * \since This function is available since SDL 3.0.0.
+ *
+ * \sa SDL_GetPerformanceFrequency
+ */
+extern DECLSPEC Uint64 SDLCALL SDL_GetPerformanceCounter(void);
+
+/**
+ * Get the count per second of the high resolution counter.
+ *
+ * \returns a platform-specific count per second.
+ *
+ * \since This function is available since SDL 3.0.0.
+ *
+ * \sa SDL_GetPerformanceCounter
+ */
+extern DECLSPEC Uint64 SDLCALL SDL_GetPerformanceFrequency(void);
+
+/**
+ * Wait a specified number of milliseconds before returning.
+ *
+ * This function waits a specified number of milliseconds before returning. It
+ * waits at least the specified time, but possibly longer due to OS
+ * scheduling.
+ *
+ * \param ms the number of milliseconds to delay
+ *
+ * \since This function is available since SDL 3.0.0.
+ */
+extern DECLSPEC void SDLCALL SDL_Delay(Uint32 ms);
+
+/**
+ * Function prototype for the timer callback function.
+ *
+ * The callback function is passed the current timer interval and returns
+ * the next timer interval. If the returned value is the same as the one
+ * passed in, the periodic alarm continues, otherwise a new alarm is
+ * scheduled. If the callback returns 0, the periodic alarm is cancelled.
+ */
+typedef Uint32 (SDLCALL * SDL_TimerCallback) (Uint32 interval, void *param);
+
+/**
+ * Definition of the timer ID type.
+ */
+typedef int SDL_TimerID;
+
+/**
+ * Call a callback function at a future time.
+ *
+ * If you use this function, you must pass `SDL_INIT_TIMER` to SDL_Init().
+ *
+ * The callback function is passed the current timer interval and the user
+ * supplied parameter from the SDL_AddTimer() call and should return the next
+ * timer interval. If the value returned from the callback is 0, the timer is
+ * canceled.
+ *
+ * The callback is run on a separate thread.
+ *
+ * Timers take into account the amount of time it took to execute the
+ * callback. For example, if the callback took 250 ms to execute and returned
+ * 1000 (ms), the timer would only wait another 750 ms before its next
+ * iteration.
+ *
+ * Timing may be inexact due to OS scheduling. Be sure to note the current
+ * time with SDL_GetTicks() or SDL_GetPerformanceCounter() in case your
+ * callback needs to adjust for variances.
+ *
+ * \param interval the timer delay, in milliseconds, passed to `callback`
+ * \param callback the SDL_TimerCallback function to call when the specified
+ *                 `interval` elapses
+ * \param param a pointer that is passed to `callback`
+ * \returns a timer ID or 0 if an error occurs; call SDL_GetError() for more
+ *          information.
+ *
+ * \since This function is available since SDL 3.0.0.
+ *
+ * \sa SDL_RemoveTimer
+ */
+extern DECLSPEC SDL_TimerID SDLCALL SDL_AddTimer(Uint32 interval,
+                                                 SDL_TimerCallback callback,
+                                                 void *param);
+
+/**
+ * Remove a timer created with SDL_AddTimer().
+ *
+ * \param id the ID of the timer to remove
+ * \returns SDL_TRUE if the timer is removed or SDL_FALSE if the timer wasn't
+ *          found.
+ *
+ * \since This function is available since SDL 3.0.0.
+ *
+ * \sa SDL_AddTimer
+ */
+extern DECLSPEC SDL_bool SDLCALL SDL_RemoveTimer(SDL_TimerID id);
+
+
+/* Ends C function definitions when using C++ */
+#ifdef __cplusplus
+}
+#endif
+#include <SDL3/close_code.h>
+
+#endif /* SDL_timer_h_ */
+
+/* vi: set ts=4 sw=4 expandtab: */