linux-pipewire: Use premultiplied alpha

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).