[platform/virtual] Drop support for vgem and rendernode
Summary:
Instead we depend on the surfaceless platform for which we recently
added support. Thus the plugin does not need to use gbm and udev
anymore. So simplifies a lot.
Test Plan: ctest (prior to breaking change) passes
Reviewers: KWin
Subscribers: kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D18160