General improvements for user custom event registration

* Switched event type to enum (int32)
* Switched polling by mask to polling by type range
* Added SDL_RegisterEvents() to allow dynamic user event registration
* Spread events out to allow inserting new related events without breaking binary compatibility
* Added padding to event structures so they're the same size regardless of 32-bit compiler structure packing settings
* Split SDL_HasEvent() to SDL_HasEvent() for a single event and SDL_HasEvents() for a range of events
* Added SDL_GetEventState() as a shortcut for SDL_EventState(X, SDL_QUERY)
* Added SDL_FlushEvent() and SDL_FlushEvents() to clear events from the event queue

src/video/win32/SDL_win32events.c

@@ -109,7 +109,7 @@ WIN_WindowProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam)
     LRESULT returnCode = -1;
 
     /* Send a SDL_SYSWMEVENT if the application wants them */
-    if (SDL_ProcessEvents[SDL_SYSWMEVENT] == SDL_ENABLE) {
+    if (SDL_GetEventState(SDL_SYSWMEVENT) == SDL_ENABLE) {
         SDL_SysWMmsg wmmsg;
 
         SDL_VERSION(&wmmsg.version);