[effects/windowaperture] Hide windows in corners

Reducing the oppacity of these windows causes them to appear as phantoms in the corners of the screen. They look quite odd on top of anything else in the corners of the screen, such as any Plasma panels.

However making them 100% opaque doesn;t work either, since then they look interactive, but they're not, and can't be made so due to the scripting API used here.

So our only real option to remove the weirdness is to make them 100% transparent, and therefore show the entirety of the desktop with no sign that the effect is in use.

simplescreenrecorder-2021-04-22_13.43.28

cc @teams/vdg