dav1dplay: Only repaint the window when necessary

The current playback loop triggers a repaint on any single event,
including spammy events such as SDL_MOUSEMOTION.

Fix this by only repainting on SDL_WINDOWEVENT_EXPOSED, which is defined
as the event sent when the window was damaged and needs to be repainted,
as well as on new frames.

Fixes videolan/dav1d#356