screen-cast: Fix monitor recording on HiDPI [gnome-3-30 backport]
It scaled the logical monitor rect with scale to get the stream dimensions, but that is only valid when having 'scale-monitor-framebuffers' enabled. Even when it was, it didn't work properly, as clutter_stage_capture_into() doesn't work properly with scaled monitor framebuffers yet.
(cherry picked from commit 3fa6a92c)