shaders/icc: add 3DLUT caching API

It's a bit clumsy to mix this into pl_icc_params, but doing it here quite elegantly avoids the need to have to plumb this through the pl_renderer somehow, because the pl_icc_params are already exposed to the user via pl_render_params.

Closes: #205 (closed) Closes: https://github.com/haasn/libplacebo/issues/139