Increased size of video buffer to prevent problems with some videos with big delay between sound and video

Modified AV-Sync code to fix a problem described in this thread.

It should help when the following condition occurs:

"Audio 686 ms behind video but already 75 video frames queued. AV-Sync might be broken. "

http://www.avenard.org/files/media/vdpau/mythtv-fixes-glvdpau20405-5.patch.gz