qt: restore window backdrop blur effect in CompositorX11

CompositorX11, unlike others, requires explicit code to adjust backdrop blur. When the effect gets activated at a later time after initialization, it currently does not have code to do the required adjustment.

This is a regression since b1e11641. I did not notice it because I was using extended frame with CSD, where this regression is masked.

Regression since !6196 (merged).

Request review @chub.