[effects/invert] Don't block blur and background contrast effect
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.
Reviewers: KWin, davidedmundson
Reviewed By: KWin, davidedmundson
Differential Revision: https://phabricator.kde.org/D16358