Summary:
This matches the DRM backend more closely and allows mid-test removal and
addition of virtual outputs with different properties in the future.
Test Plan: Before and after 93% tests passed.
Reviewers: #kwin, graesslin
Reviewed By: #kwin, graesslin
Subscribers: graesslin, kwin, #kwin
Tags: #kwin
Differential Revision: https://phabricator.kde.org/D11351
