This covers Profiles 5 and 8.X. After playing around with the
implementation quite a lot, I eventually settled on the current state,
which requires no LUTs or state and is implemented entirely in pure
GLSL. This is slightly slower than a pure 1D LUT approach, but has the
benefit of not needing to add an extra `pl_shader_obj` to the API at
all, and thus being transparently backwards compatible.

The decoding metadata is assumed to be passed in via a new struct
`pl_dovi_metadata`, which contains both the colorspace matrices
(especially important for profiles 8.X) and reshaping coefficients.

Closes #113