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.

include/SDL_timer.h

@@ -42,6 +42,10 @@ extern "C" {
  *
  * This value wraps if the program runs for more than ~49 days.
  *
+ * \deprecated This function is deprecated as of SDL 2.0.18; use
+ *             SDL_GetTicks64() instead, where the value doesn't wrap
+ *             every ~49 days.
+ *
  * \returns an unsigned 32-bit value representing the number of milliseconds
  *          since the SDL library initialized.
  *
@@ -49,15 +53,42 @@ extern "C" {
  *
  * \sa SDL_TICKS_PASSED
  */
-extern DECLSPEC Uint32 SDLCALL SDL_GetTicks(void);
+extern SDL_DEPRECATED DECLSPEC Uint32 SDLCALL SDL_GetTicks(void);
 
 /**
- * Compare SDL ticks values, and return true if `A` has passed `B`.
+ * 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. 64-bit values can just be safely
+ * compared directly.
  *
  * For example, if you want to wait 100 ms, you could do this:
  *
- * ```c++
- * Uint32 timeout = SDL_GetTicks() + 100;
+ * ```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.
+ */
+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, 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
  * }