[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

plugins/kdecorations/aurorae/src/aurorae.cpp

@@ -337,6 +337,7 @@ void Decoration::init()
         if (usingGL) {
             // first create the context
             QSurfaceFormat format;
+            format.setSwapBehavior(QSurfaceFormat::SingleBuffer);
             format.setDepthBufferSize(16);
             format.setStencilBufferSize(8);
             m_context.reset(new QOpenGLContext);