Added a macro SDL_TICKS_PASSED() to correctly compare two 32-bit tick values.

Went through the code and used the macro and fixed a couple places that were using incorrect timestamp comparisons.

include/SDL_timer.h

@@ -44,6 +44,17 @@ extern "C" {
  */
 extern DECLSPEC Uint32 SDLCALL SDL_GetTicks(void);
 
+/**
+ * \brief Compare SDL ticks values, and return true if A has passed B
+ *
+ * e.g. if you want to wait 100 ms, you could do this:
+ *  Uint32 timeout = SDL_GetTicks() + 100;
+ *  while (!SDL_TICKS_PASSED(SDL_GetTicks(), timeout)) {
+ *      ... do work until timeout has elapsed
+ *  }
+ */
+#define SDL_TICKS_PASSED(A, B)  ((Sint32)((B) - (A)) <= 0)
+
 /**
  * \brief Get the current value of the high resolution counter
  */