shaders/icc: support forced black point linearization

This overrides the built-in BPC logic and progressively mixes a linear ramp into the near-black entries of the generated LUT. May help fix "bad" monitor profiles, containing invalid colorimetric tables.

Disabled by default because it effectively disables the 3DLUT near black, which may not be desired, especially for well-behaved profiles.

Closes: #219 (closed)