ffplay: remove refresh thread to achieve more precise frame delay

We use a refresh loop which displays the frames and also does the polling for
pending events in a non-blocking way. If we know the required delay until the
next frame, then exactly that amount of sleeping will be done. After each
handled event we check if we have to display a frame which kind of makes
displaying the frame a high priority event.

This improves greatly the smoothness of the video output especially with 50fps
content.

Signed-off-by: Marton Balint <cus@passwd.hu>