screenshot: Grab screenshot during paint on X11 for area type too

Turns out that shell_screenshot_screenshot_area was affected
by the same issue that shell_screenshot_screenshot used to have.
Adapting code changes from commit id c09be8b0:
"screenshot: Grab screenshot during paint on X11" fixes it.

(cherry picked from commit 1989a2f9)

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1552>