blur-effect: Don't use stage view when drawing off-stage

When we're painting off-stage, for example because we're screencasting
or taking a screenshot, there won't be a stage-view associated with the
paint context. The BlurEffect previously didn't handle that case and
would crash.

Fix that and handle that case by assuming the scale is 1 and not
offsetting the rectangle we blit from the draw framebuffer.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1673>

(cherry picked from commit 09602ae2)