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)