PGS subtitles vanished or displaced each other when a display set put two
images on screen at once (dialogue plus a sign or song caption), and
palette-only fade updates blanked the subtitle entirely. media3's PgsParser
keeps one bitmap buffer and only the first composition object's coordinates,
and it discards all state between display sets.
Replace it with PgsCompositionParser, a port of FFmpeg's pgssubdec model:
epoch-scoped object/palette caches keyed by id, in-place palette updates,
one cue per composition object reference, and limited-range BT.709/BT.601
color conversion selected by plane height.
close#1953
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