screenshot: Add API to screenshot stage to a ClutterContent

screenshot: Add API to screenshot stage to a ClutterContent

As mentioned in the last commit, we'll split up taking screenshots into
creating a GPU texture first, and later saving that to disk as a PNG.

For individual windows it's already easy to get a ClutterContent with
the texture using meta_window_actor_paint_to_content (), for the stage
it's not that easy and involves a few extra steps including X11 specific
ones.

So introduce a new ShellScreenshot API which does all that and provides
the caller with a ClutterContent of the stage:
shell_screenshot_screenshot_stage_to_content ()

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1954>