Fix HSY selector modes

The RGB<=>HSY conversion really only works properly in linear RGB.
Gamma needs to be applied to Y', and is now taken from Advanced Color Selector
configuration.

Luma coefficients are now fetched on reconfiguration instead of every single
color conversion.

Caveat: Some color profiles like "ITUR_2100_PQ_FULL.ICC" define no colorants
and hence they cannot be linearized with the current implementation.
A substitution of color spaces like the Small Color Selector does seems
unavoidable in the long run...