Synchronize number of stamps to number of h264 frames

[proger]

1. remove all packets from the stream that have no decodable images in them (bad idea: see below)
2. observe that some streams start with leading P frames which we can't use, so we drop them
3. add debug output to ffmpeg to see discrepancies in input vs output frame counts

Thanks @nicholas-carlotti for testing

[proger]

I found that dropping packets that don't have decodable frames by libmedia_codec have a negative impact on the video output.