[css-color] blending options

[svgeesus]

https://drafts.csswg.org/css-color/#blend-adjuster

This blends the two colors in a way that pays attention to alpha, similar to how compositing does. Is there a better formula? The current one was determined empirically to give good results, but isn’t motivated by any theory.

[upsuper]

The alpha channel of the result color being the average of the two colors looks pretty weird. Should that be the linear interpolation given the percentage? The current algorithm means color(color_a blenda(color_b 100%)) may not be color_a and color(color_a blenda(color_b 0%)) may not be color_b.

[svgeesus]

See also #445

[svgeesus]

This section was ong ago removed, so the issue is moot.