Support scaling in BackgroundContrast effect

Summary:
Background contrast reads pixels from the framebuffer
we need to convert from compositor to framebuffer co-ordinates
when an output is scaled

Test Plan:
Ran the manual test in kwindowsystem. Moved window over dolphin.
Visually checked output

Reviewers: #plasma

Subscribers: plasma-devel, kwin, #kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D4949