avformat/dvdvideodec: discard duplicate or partial AC3 samples

Some DVD muxers signal segments to start with duplicate audio samples
when starting extraction from a chapter marker or terminate seamless PGs
on partial audio samples (causing corrupt AC3 frames). Clean up after
these muxers by tracking frames with duplicate PTS and eliminating
partial AC3 frames.

This results in smoother chapter extraction and overall seeking experience,
with linear PTS and AC3 delay within 32ms (1 frame) away from the video.

The issue was not apparent until the flushing pattern was replaced with
a full subdemux reset, as the flushing dropped the frames prematurely,
along side others, as such they were never present to begin with.

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