zimg: add support for some RGB fringe formats

This covers 8 and 16 bit packed RGB formats. It doesn't really help with
any actual use-cases, other than giving the finger to libswscale.

One problem is with different color depths. For example, rgb565 provides
1 bit more resolution to the green channel. zimg can only dither to a
uniform depth. I tried dithering to the highest depth and shifting away
1 bit for the lower channels, but that looked ugly (or I messed up
somewhere), so instead it dithers to the lowest depth, and adjusts the
value range if needed. Testing with bgr4_byte (extreme case with 1/2/1
depths), it looks more "grainy" (ordered dithering artifacts) than
libswscale, but it also looks cleaner and smoother. It doesn't have
libswscale's weird red-shift. So I call it a success.

Big endian formats need to be handled explicitly; the generic big endian
swapper code assumes byte-aligned components.

Unpacking is done with shifts and 3 LUTs. This is symmetric to the
packer. Using a generated palette might be better, but I preferred to
keep the symmetry, and not having to mess with a generated palette and
the pal8 code.

This uses FFmepg pixfmts constants directly. I would have preferred
keeping zimg completely separate. But neither do I want to add an IMGFMT
alias for every of these formats, nor do I want to extend our imgfmt
code such that it can provide a complete description of each packed RGB
format (similar to FFmpeg pixdesc).

It also appears that FFmpeg pixdesc as well as the FFmpeg pixfmt doxygen
have an error regarding RGB8: the R/B bit depths are swapped. libswscale
appears to be handling them differently. Not completely sure, as this is
the only packed format case with R/B havuing different depths (instead
of G, the middle component, where things are symmetric).

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