ffplay: add delay multiple times to frame_timer if it is less than current time

ffplay: add delay multiple times to frame_timer if it is less than current time

If the picture queue is empty, or when the calculated delay is 0, frame_timer
is not increased but we are still displaying the old frame. When we eventually
get a frame, where the computed delay is positive, so we don't need to drop any
more frames, then it is best to update frame_timer to be as near as the current
time as it can.

This way we dont't have to wait several frames to add the necesarry delays to
frame_timer to reach current time, therefore there are no extra frame drops
after reaching a positive delay.

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