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_error.h

@@ -0,0 +1,163 @@
+/*
+  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.
+*/
+
+/**
+ *  \file SDL_error.h
+ *
+ *  Simple error message routines for SDL.
+ */
+
+#ifndef SDL_error_h_
+#define SDL_error_h_
+
+#include <SDL3/SDL_stdinc.h>
+
+#include <SDL3/begin_code.h>
+/* Set up for C function definitions, even when using C++ */
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/* Public functions */
+
+
+/**
+ * Set the SDL error message for the current thread.
+ *
+ * Calling this function will replace any previous error message that was set.
+ *
+ * This function always returns -1, since SDL frequently uses -1 to signify an
+ * failing result, leading to this idiom:
+ *
+ * ```c
+ * if (error_code) {
+ *     return SDL_SetError("This operation has failed: %d", error_code);
+ * }
+ * ```
+ *
+ * \param fmt a printf()-style message format string
+ * \param ... additional parameters matching % tokens in the `fmt` string, if
+ *            any
+ * \returns always -1.
+ *
+ * \since This function is available since SDL 3.0.0.
+ *
+ * \sa SDL_ClearError
+ * \sa SDL_GetError
+ */
+extern DECLSPEC int SDLCALL SDL_SetError(SDL_PRINTF_FORMAT_STRING const char *fmt, ...) SDL_PRINTF_VARARG_FUNC(1);
+
+/**
+ * Retrieve a message about the last error that occurred on the current
+ * thread.
+ *
+ * It is possible for multiple errors to occur before calling SDL_GetError().
+ * Only the last error is returned.
+ *
+ * The message is only applicable when an SDL function has signaled an error.
+ * You must check the return values of SDL function calls to determine when to
+ * appropriately call SDL_GetError(). You should *not* use the results of
+ * SDL_GetError() to decide if an error has occurred! Sometimes SDL will set
+ * an error string even when reporting success.
+ *
+ * SDL will *not* clear the error string for successful API calls. You *must*
+ * check return values for failure cases before you can assume the error
+ * string applies.
+ *
+ * Error strings are set per-thread, so an error set in a different thread
+ * will not interfere with the current thread's operation.
+ *
+ * The returned string is internally allocated and must not be freed by the
+ * application.
+ *
+ * \returns a message with information about the specific error that occurred,
+ *          or an empty string if there hasn't been an error message set since
+ *          the last call to SDL_ClearError(). The message is only applicable
+ *          when an SDL function has signaled an error. You must check the
+ *          return values of SDL function calls to determine when to
+ *          appropriately call SDL_GetError().
+ *
+ * \since This function is available since SDL 3.0.0.
+ *
+ * \sa SDL_ClearError
+ * \sa SDL_SetError
+ */
+extern DECLSPEC const char *SDLCALL SDL_GetError(void);
+
+/**
+ * Get the last error message that was set for the current thread.
+ *
+ * This allows the caller to copy the error string into a provided buffer, but
+ * otherwise operates exactly the same as SDL_GetError().
+ *
+ * \param errstr A buffer to fill with the last error message that was set for
+ *               the current thread
+ * \param maxlen The size of the buffer pointed to by the errstr parameter
+ * \returns the pointer passed in as the `errstr` parameter.
+ *
+ * \since This function is available since SDL 3.0.0.
+ *
+ * \sa SDL_GetError
+ */
+extern DECLSPEC char * SDLCALL SDL_GetErrorMsg(char *errstr, int maxlen);
+
+/**
+ * Clear any previous error message for this thread.
+ *
+ * \since This function is available since SDL 3.0.0.
+ *
+ * \sa SDL_GetError
+ * \sa SDL_SetError
+ */
+extern DECLSPEC void SDLCALL SDL_ClearError(void);
+
+/**
+ *  \name Internal error functions
+ *
+ *  \internal
+ *  Private error reporting function - used internally.
+ */
+/* @{ */
+#define SDL_OutOfMemory()   SDL_Error(SDL_ENOMEM)
+#define SDL_Unsupported()   SDL_Error(SDL_UNSUPPORTED)
+#define SDL_InvalidParamError(param)    SDL_SetError("Parameter '%s' is invalid", (param))
+typedef enum
+{
+    SDL_ENOMEM,
+    SDL_EFREAD,
+    SDL_EFWRITE,
+    SDL_EFSEEK,
+    SDL_UNSUPPORTED,
+    SDL_LASTERROR
+} SDL_errorcode;
+/* SDL_Error() unconditionally returns -1. */
+extern DECLSPEC int SDLCALL SDL_Error(SDL_errorcode code);
+/* @} *//* Internal error functions */
+
+/* Ends C function definitions when using C++ */
+#ifdef __cplusplus
+}
+#endif
+#include <SDL3/close_code.h>
+
+#endif /* SDL_error_h_ */
+
+/* vi: set ts=4 sw=4 expandtab: */