This partiular format is not marked as AV_PIX_FMT_FLAG_RGB in FFmpeg's
pixdesc table, so mpv assumed it's a YUV format.
This is a regression, since the old code in mp_imgfmt_get_desc() also
treated this format specially to avoid this problem. Another format
which was special-cased in the old code was AV_PIX_FMT_MONOBLACK, so
make an exception for it as well.
Maybe this problem could be avoided by mp_image_params_guess_csp() not
forcing certain colorimetric parameters by the implied colorspace, but
certainly that would cause other problems. At least there are mistagged
files out there that would break. (Do we actually care?)
Fixes#4965.
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