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: videolan/libplacebo#205
Closes: https://github.com/haasn/libplacebo/issues/139