Group the variable state up into its own struct, which also includes
metadata like whether or not the variable is dynamic.

Instead of hard-coding static names for all variables, generate fresh
identifiers and pass them around. (In theory, we could get away with
just the `_N` variable names, but I want to keep around the
human-friendly variable names for debugging purposes; so I went with the
`_name_N` approach)

Also change the ra_var interpretation back from column major to row
major, to match the struct color_matrix/color_transform. This is
probably more convenient for everybody involved, even though it makes
`dim_m` sort of weird. The new helper functions should hopefully make
that less painful anyway.

As an aside, fixes a bug where the wrong luma coefficients were used for
the inverse OOTF. Annoyingly, this makes it harder to detect when
src_luma/dst_luma are required, so just always load the uniforms again.
(It's not like it matters)