plugins/screenshot: re-render the screen instead of just blitting

This allows us to skip intermediary colorspace transformations, which avoids information loss with night light, and allows for tone mapping of HDR content to be applied for each window as needed.

BUG: 498881

Requires spectacle!474 (merged) for spectacle to still work

Draft because I don't know how to test the area mode - Spectacle doesn't seem to use it?

Edit: Flameshot uses it, works fine.