timer: Added SDL_GetTicks64(), for a timer that doesn't wrap every ~49 days.

Note that this removes the timeGetTime() fallback on Windows; it is a
32-bit counter and SDL2 should never choose to use it, as it only is needed
if QueryPerformanceCounter() isn't available, and QPC is _always_ available
on Windows XP and later.

OS/2 has a similar situation, but since it isn't clear to me that similar
promises can be made about DosTmrQueryTime() even in modern times, I decided
to leave the fallback in, with some heroic measures added to try to provide a
true 64-bit tick counter despite the 49-day wraparound. That approach can
migrate to Windows too, if we discover some truly broken install that doesn't
have QPC and still depends on timeGetTime().

Fixes #4870.

src/timer/SDL_timer.c

@@ -370,4 +370,14 @@ SDL_RemoveTimer(SDL_TimerID id)
     return canceled;
 }
 
+/* This is a legacy support function; SDL_GetTicks() returns a Uint32,
+   which wraps back to zero every ~49 days. The newer SDL_GetTicks64()
+   doesn't have this problem, so we just wrap that function and clamp to
+   the low 32-bits for binary compatibility. */
+Uint32
+SDL_GetTicks(void)
+{
+    return (Uint32) (SDL_GetTicks64() & 0xFFFFFFFF);
+}
+
 /* vi: set ts=4 sw=4 expandtab: */