Qt: Frosted glass effect does not have proper blending

Frosted glass effect is currently applied on top of the rendered source item. This obviously causes intended blending to fail.

The only solution I see is enabling clipping in the source item while the effect is active. So that the effect can be blended correctly with what's in the background of the source item, like the rest of the source item.

The effect should probably also have its opacity bound to source item opacity.