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.

!415
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Status Job ID Name Coverage
  Build
passed #196909
build-mutter

00:02:02

 
  Test
passed #196910
test-mutter

00:03:02