It's conceptually broken for an hw format to be identified as a
supported sw format for an hwdec, but in the case of the drm hwdec,
we have no choice but to do so for the weird rpi formats, as these
are declared, incorrectly, as hw formats in their forked ffmpegs.
This means we can't reject such formats as a matter of principle,
as we do today. Instead let's just assume that such formats can always
be accepted as-is, and will never require conversion. In practice, this
is either true or it will fail in the VO, which is the old behaviour
from before I introduced the conversion filter capability.
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