[RFC] vulkan: compensate alpha to better match perceptual blend intent

The Vulkan renderer performs alpha blending in linear RGB space, which preserves hue better than blending sRGB-encoded values directly (as the gles and pixman renderers do), but unfortunately tends to give a too-bright result when blending dark and light colors.

(In desktop usage, this especially affects dark, semi-transparent tooltips, which appear significantly more transparent than expected, affecting readability if light text underneath shows through.)

This is a novel (I think) approach to compensating for this effect by adjusting the alpha value of the source texture - basically the result is that dark semi-transparent pixels are made a little more opaque, while light semi-transparent pixels are made a little more transparent. Alpha values of 0 and 1 are unchanged.

I am somewhat new to science of color blending (Björn Ottosson's page, "How software gets color wrong" is very enlightening) but I think this approach makes at least a little bit of sense theoretically, and the result seems to me subjectively to be an improvement.

Analysis from an expert on the subject would be greatly appreciated.