mov/mp4 muxers rewrite the stream time base at write_header (mov timescale
>= 10000) and FFmpeg 9 no longer rescales packets for us, so mpeg2video
clips were muxed with pts in encoder ticks -- a 10-frame/10fps clip became
1ms long and every seek past t=0 decoded to the EOF frame.
- read back the real stream time base after write_header and rescale
packets (including the flush path) before write_interleaved
- warn once when retrieve_frame's EOF fallback returns a frame far from
the requested timestamp
- testmedia round-trip now asserts container duration and per-frame
decode instead of only t=0
Mike-Solar
9f88b9b856