Make sure any stored packets are flushed when the DVDRingBuffer needs to wait for the player.

This fixes playback issues with still frames when using VDPAU (or when the 'Extra audio buffering' option is enabled) due to the still frame getting stuck in the decoder's buffers and the player running out of frames.
Fixed flickering playback when transistioning from/to a still frame with OpenGL playback.  The underlying issue (paused video not being unpaused) occured with VDPAU playback too but did not seem to cause any visible issues.