shaders/icc: completely refactor

Rewrite all of the ICC profile handling code, fundamentally getting rid
of the 'ICC->ICC transform' design and representing each profile as an
independent object that can be either decoded or encoded, at separate
stages in the pipeline.

This is only an initial version, and serves to establish the API. It is
not yet fully optimized, but optimizations can and will be applied
transparently underneath the hood of this API (e.g. skipping the full
3DLUT when not required, implementing support for fixed function
profiles, natively applying matrix+shaper profiles, etc.)

This design brings with it a number of advantages, most notably allowing
support for using libplacebo-native tone mapping together with ICC
profiles, by properly forwarding the actual contrast values detected
from the profile.

In addition to a redesign of the core logic, also changes things so that
the ICC.h header is always installed, even if non-functional -
`pl_icc_open` will simply always return NULL.