This refactors the way we store data like scaler LUTs, with the intent
of generalizing it for other types of LUTs (e.g. dither matrix) in an
easy-to-use but flexible way.

Instead of trying to come up with sane semantics for who gets to use
what fields inside a pl_shader_obj, we just abstract it away and let the
caller deal with the *priv representation (as usual). Unfortunately,
this requires "nesting" pl_shader_objs for cases like the sampler LUTs
where we need to store information about the scaling kernel *and* the
underlying LUT. But that's not too bad, I think.

I had to rewrite the code that uses pl_shader_objs. Hopefully for the
better.