plugins/blur: Fix blurred region sticking outside panel popups

The blur geometry is scaled in the global coordinate space, while it
works fine with integer scale factors, it's not okay with fractional
scale factors as it doesn't match how the ItemRenderer snaps quads to
the pixel grid.

Note that blur behind apps can be still off by one pixel, but it
should be harder to notice it. In order to fix it, it would be great
to apply effects behind Items, which is on my todo list.


(cherry picked from commit 9abf7a9d)