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