lavd/sdl: add event handler thread

SDL_Init() is called on the event handler thread, as required by SDL in
Windows to avoid deadlocks as discovered by Roger Pack.

Fix trac ticket #1743 and #1744.
lavd/sdl: factorize overlay rect size in a separate function
lavd/sdl: move compute_overlay_rect() before event_thread()

It will be used in event_thread() in a pending patch.
lavd/sdl: allow to change window size
lavd/sdl: apply misc cosmetics to options
lavd/sdl: add delay when no events are found in the event queue

Avoid busy loops and decrease CPU usage.

Suggested-By: Roger Pack