ARGB buffers are premultiplied
D7460 in kwayland assumes all WL_SHM_FORMAT_ARGB8888 buffers have
opacity premultipied RGB values.
Kwin tests need updating to do the same.
Rendering code did not need changing.
Test Plan: Tests now pass
Reviewers: Plasma, graesslin
Reviewed By: Plasma, graesslin
Subscribers: plasma-devel, kwin, KWin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D7607