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Subsurface blending regression from !596

The change in !596 (9a6a4e70) apparently broke previous behaviour of blending. This is prominently visible with the Gstreamer Waylandsink example demos.

main: Screenshot_from_2022-01-10_17-39-25

previously and other compositors: Screenshot_from_2022-01-10_17-38-43

Note: the "video" here contains alpha/is not opaque. Its surface is on top of a black background surface.

Also: the Gstreamer Waylandsink appears to be mainly used for cases where the surface can be scanned out by the display controller. So this might be an example of display controllers making assumptions about non-linear blending, when done in hardware and not by the shader.

cc @pq, @daniels

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