Pipewire desktop portal capture assumes premultiplied alpha, so make
sure to use the right blend equation to make translucent pixels work
correctly.
This is still broken for emissive pixels (alpha < color) since OBS seems
to unpremultiply at some point during blending, but it works properly
for translucent pixels (emissive pixel support requires an end-to-end
premultiplied pipeline).
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