Convert virtual backend to per screen rendering
Summary:
Output scaling can't ever work on a single buffer; especially if they're
different scales and overlapping.
This ports the virtual backend to perScreenRendering so that I can use
it for
tests.
ctest fails here, but it fails on the tests that it failed on before..
Reviewers: Plasma
Subscribers: plasma-devel, kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D3306