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