colorspace: add support for Dolby Vision

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.

TODO:

• test MMR code
• clean up debug crap
• optimize/benchmark MMR
• add GLSL version checks for floatBitsToUint (or work around?)

Closes #113 (closed)