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.
v2: compensate alpha in solid color conversions also
v3: un-premultiply average value for solid color conversion
