If we have a window that match the size (i.e. will pass the "fits
framebuffer" low level check), that doesn't mean it matches the
position. For example, if we have two monitors 2K monitors, with two 2K
sized windows, one on monitor A, and one on monitor both monitor A and
B, overlapping both, if the latter window is above the former, it'll end
up bing scanned out on both if it ends up fitting all the other
requirements.
Fix this by checking that the paint box matches the stage view layout,
as that makes sure the actor we're painting isn't just partially on the
right view.
Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/2387
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2624>
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