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.

Fixes #3301 (closed)