Desktop: Fix UI and viewport blending

[timon-schelling]

Closes #3043

[TrueDoctor]

This currently applies srg gamma conversions multiple times. Shouldn't we just create a srgb version of the viewport value and keep the others as is?

[timon-schelling]

Sure, found a way of writing that relatively clean.

Sadly this doesn't active the exact result like web. With "fade artwork" at 80% white color artwork results in #4E4E4E in web and #4C4C4C in desktop.

There is also a problem with the dots. Node graph view background dots on .graph:before are not transparent when rendered with hole punch enabled. "mix-blend-mode: screen" doesn't work like in web in that case. Cef returns [34, 34, 34, 255] (alpha always 255) for these dots when fade artwork is 80%.

[timon-schelling]

[timon-schelling]

If I understand the chromium docs correctly it will do blending in the renderer’s working/display color space (not strictly sRGB) with source-over (Porter-Duff).
This PR tries to do the same. Result is not quit the same, slight visual change when disabling vello.

[Keavon]

You'd mentioned the blend mode on the parts of the graph. That won't be something we have any control over, since blend modes fundamentally have no effect at the bottom of a compositing stack. We'll have to wait for our custom-rendered node graph UI overlay before we can get that to draw similarly, but we can just make it look as decent as possible in the meantime.

[TrueDoctor]

Is this ready for review / merge then?

[timon-schelling]

Yes, the issue with the dots should be solved separately, should only concern frontend code.

[timon-schelling]

Just noticed that overlays look more like in web when bleeding overlays and viewport in linear and then bleding that result in srgb with the ui.
Also added a early return for the common case were overlays and ui are transparent.

[timon-schelling]

@TrueDoctor ready for final review and merge :)