Tighten up the sdl_ufunc_ret semaphore, ensure it's not already posted

before waiting for it.

Flush doesn't need to sdl_ufunc_ret semaphore anymore since it places
rectangles back in the list on failure

Only enable SDL_SYSWMEVENT (a main cause of issues) during copy/paste
operations.  This should mostly elimiate timeouts and odd 1-second pasues
during initialization on Linux.