screen-cast: Fix monitor recording on HiDPI

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.

GNOME/mutter!415
2 jobs for wip/screen-cast-hidpi in 5 minutes and 14 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #196900
build-mutter

00:01:45

 
  Test
passed #196901
test-mutter

00:03:29