They were used at some point, but then fell into disuse. In general,
these old flags are all a bit fuzzy, so it's a good idea to remove them
as much as possible.
The comment about MP_IMGFLAG_PAL isn't true anymore. The old meaning was
deprecated at some point, and the flag was removed from "pseudo
paletted" formats. I think mpv at one point changed its own flag from
AV_PIX_FMT_FLAG_PSEUDOPAL to AV_PIX_FMT_FLAG_PAL, when the former was
deprecated, and it became unnecessary to allocate a palette for
non-paletted formats. (The one who deprecated in FFmpeg was me, if you
wonder.)
MP_IMGFLAG_PLANAR was used in command.c, use a relatively similar flag
as replacement.
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