[Aurorae] Remove effective triple buffering

Summary:
KWin blits contents into a QImage raster buffer. Because of this we
don't need our underlying GL surface to be double buffered as it
effectively makes the whole thing tripple buffered.

Test Plan: Ran Aurorae, looked the same, should save some memory

Reviewers: KWin, zzag

Reviewed By: KWin, zzag

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D23331