plugins/screenshot: Fix monitor screenshots on x11
After some refactoring changes, m_paintedScreen is always the first output on x11, so it's no longer possible to take screenshots of other outputs.
(cherry picked from commit bc29e09d)
After some refactoring changes, m_paintedScreen is always the first output on x11, so it's no longer possible to take screenshots of other outputs.
(cherry picked from commit bc29e09d)