[effects/invert] Don't block blur and background contrast effect

Summary:
Background behind some windows is not blurred because the invert effect
sets PAINT_WINDOW_TRANSFORMED. This essentially "turns off" the blur and
the background contrast effect unless we set the force roles.

Because the invert effect is a "post-processing"(not really) effect we
don't have to set that flag.

BUG: 279076
BUG: 359583
FIXED-IN: 5.14.3

Test Plan: {F6341916}

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D16358