render/pixman: doesn't clear transparent regions

[emersion]

1. Start Sway with the Pixman renderer
2. Start slurp
3. Start selecting a region
4. The selected region is full of previous rectangles. The area which isn't selected is supposed to be semi-transparent, but it's completely white.

cc @bl4ckb0ne

[bl4ckb0ne]

I managed to reproduce on my side, gonna investigate it. Could you tell me more about slurp's rendering process please? I guess it's a layer shell, but how do you handle blanking the screen and the drawing of the rectange?

[bl4ckb0ne]

I think it's because the renderer doesn't implement the texture write_pixels function.

[emersion]

Unlikely. Re-using textures ils an optional optimization irrelevant for the Pixman renderer since there's no GPU upload going on.

[emersion]

slurp just clears the whole buffer and renders the whole thing each frame. I think the Pixman renderer doesn't handle a non-opaque alpha channel properly when blending (in render_texture).