ffplay: replace SDL_delay in read thread with SDL_CondWait

ffplay: replace SDL_delay in read thread with SDL_CondWait

When the audio queue was empty, it was not filled until the 10ms delay expired
in the read thread. This patch changes the delay method with a condition wait,
which reacts to an empty queue a lot faster, therefore the audio buffer
underruns become less common especially after seeking.

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