shell-screenshot: Use mutter paint signals to optionally draw cursor

shell-screenshot: Use mutter paint signals to optionally draw cursor

Under wayland, if the cursor should be included when doing a fullscreen
screenshot, we can rely on mutter "paint" signal to have it composited for free.
Otherwise if it's not requested, we can use the "actors-painted" signal to get a
stage texture before the mouse overlay has been added.

Instead, under X11 or when only a window screenshot is requested, we still
need to draw it manually.

https://bugzilla.gnome.org/show_bug.cgi?id=765011
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/5