[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.

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

render/vulkan/shaders/texture.frag

@@ -26,11 +26,17 @@ vec4 srgb_color_to_linear(vec4 color) {
 		return vec4(0);
 	}
 	color.rgb /= color.a;
+	// Estimate perceptual lightness from sRGB values
+	float v = (color.r + color.g + color.b) / 3;
 	color.rgb = vec3(
 		srgb_channel_to_linear(color.r),
 		srgb_channel_to_linear(color.g),
 		srgb_channel_to_linear(color.b)
 	);
+	// Adjust alpha to make dark semi-transparent overlays a bit more
+	// opaque, better matching perceptual intent compared to simple
+	// linear blending (see README-alpha-blend for a longer discussion)
+	color.a = ((v - 0.5) * color.a * color.a + color.a) / (v + 0.5);
 	color.rgb *= color.a;
 	return color;
 }