Removed SDL_bool in favor of plain bool

We require stdbool.h in the build environment, so we might as well use the plain bool type.

If your environment doesn't have stdbool.h, this simple replacement will suffice:
typedef signed char bool;

src/main/SDL_main_callbacks.c

@@ -69,7 +69,7 @@ static void SDL_DispatchMainCallbackEvents(void)
     }
 }
 
-static SDL_bool SDLCALL SDL_MainCallbackEventWatcher(void *userdata, SDL_Event *event)
+static bool SDLCALL SDL_MainCallbackEventWatcher(void *userdata, SDL_Event *event)
 {
     if (ShouldDispatchImmediately(event)) {
         // Make sure any currently queued events are processed then dispatch this before continuing