[3.38] screenshot: Grab screenshot during paint on X11

We might not be able to directly paint the stage to an offscreen, if there currently is a fullscreen unredirected window. To make it possible to take a screenshot in this situation, disable unredirecting, queue a frame, and take a screenshot after having painted that frame, before we go back being unredirected.

Don't do this on Wayland because it's a waste.

Closes: mutter#1453 (closed) Part-of: !1534 (merged)

(cherry picked from commit c09be8b0)