Add something that allows is to extract the component order from various
RGBA formats. In fact, also handle YUV, GBRP, and XYZ formats with this.
It introduces a new struct mp_regular_imgfmt, that hopefully will
eventually replace struct mp_imgfmt_desc. The latter is still needed by
a lot of code though, especially generic code. Also vo_opengl still uses
the old one, so this commit is sort of incomplete.
Due to its genericness, it's also possible that this commit introduces
rendering bugs, or accepts formats it shouldn't accept.
1dffcb0167