[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