colorspace: add color temperature adjustment
In theory, this is not quite the right place to do this adjustment, because it's applied in gamma light. But the alternative fix here, I think, would be making `pl_renderer` smart enough to apply these controls on its own. Plus, I don't think this feature is worth the extra API complexity of somehow moving it to a separate location. The more we can roll into this struct, the better. Supporting this, a helper is provided to compute the CIE xy coordinates for a given color temperature, as well as a function to compute a chromatic adaptation matrix.