(WIP) shaders/colorspace: implement reshaping

Needed for Dolby Vision Profile 5 and friends. Implemented as 3x1D LUTs,
to avoid having to do any of this math in the shader. (And allow me to
be lazy and copy/paste the reference implementation)

Probably fast enough to not bother micro-optimizing further.

TODO:
- implement MMR reshaping
- signal resulting bit depth explicitly?

Closes #113