I guess I decided to stuff it all into mp_imgfmt_desc (the "old"
struct). This is probably a mistake. At first I was afraid that this
struct would get too fat (probably justified, and hereby happened), but
on the other hand mp_imgfmt_get_desc() (which builds the struct) calls
the former mp_imgfmt_get_layout(), and the separation doesn't make too
much sense anyway. Just merge them.
Still, try to keep out the extra info for packed YUV bullshit. I think
the result is OK, and there's as much information as there was before.
The test output changes a little. There's no independent bits[] array
anymore, so formats which did not previously have set this now show it.
(These formats are mpv-only and are still missing the metadata. To be
added later). Also, the output for the cursed packed formats changes.
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