[platform/virtual] Swap buffers in EGL GBM backend
Summary:
The virtual platform did not actually present content and spin the
compositor. Let's do this from now on , first for the EGL GBM backend.
Test Plan: All tests pass according to Vlad
Reviewers: KWin, zzag
Reviewed By: KWin, zzag
Subscribers: graesslin, zzag, kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D18436