Main callbacks should be processed even if events are already pending in SDL_WaitEvent()

Fixes https://github.com/libsdl-org/SDL/issues/15485

(cherry picked from commit 757400e044)

src/events/SDL_events.c

@@ -1726,6 +1726,10 @@ bool SDL_WaitEventTimeoutNS(SDL_Event *event, Sint64 timeoutNS)
         }
     } else {
         // Has existing events
+        if (!event) {
+            // The application expects callbacks to run even if events are pending
+            SDL_RunMainThreadCallbacks();
+        }
         return true;
     }
     // We should have completely handled timeoutNS == 0 above