demuxer: parser sanity checks

- added more sanity checks
- be more verbose on syncing streams
demuxer: fixed wrong MPEG2 audio header (reenables MPEG 2.5)
demuxer: reset parser buffer on overflow
demuxer: updated framesize calculation for mpeg audio
demuxer: hopefully final H264 parser fix

thanks to fernetmenta for reporting this
demuxer: divide mpeg2 video pes packet into single pictures

some streams transmit more than 1 picture per PES packet