Dubious changes to h264 parser
More WIP
tsdemux: Really go backward

Fixes infinite loops, reveals garbage seek is not completely fixed.
WIP
Revert "Dubious changes to h264 parser"

This reverts commit 97f1b4d577e446f9a0a1ec2da3cfe223b2b1ad64.
tsdemux: never set seek_offset < 0