Implement absolute color conversions and chromatic adaptation

[tomcur]

This adds absolute color conversions and chromatic adaptation, and is, more or less, a prerequisite for #137.

Conversion methods are added to ColorSpace, ColorSpaceTag, and DynamicColor, that convert between color spaces while keeping the same absolute color (i.e., if that color were to be reproduced, it would be the same physical color, but a perceptually different color under the intended reference white of the color space). These methods are suffixed _absolute. This also adds a chromatically_adapt method for manual chromatic adaptation (useful for, e.g., "white balancing" pictures).

The white points are represented as CIE xy chromaticities. Calculation of chromatic adaptation matrices is const where possible.

A follow-up to this would be to manually implement ColorSpace::{to_linear_srgb_absolute, from_linear_srgb_absolute, convert} for the color spaces we provide. Those can in most cases just lift directly from their non-_absolute counterparts, with the exceptions being XYZ-D50, ACEScg and ACES2065-1. Those methods will get somewhat simpler matrices, as the {D50, ACES}<->D65 adaptation transforms can be dropped.

[raphlinus]

This looks great, thanks! I tried to find a nit just to prove I actually read it, but failed to do so.