Use glFlush() if swapBehavior is single-buffered

Otherwise, use swapBuffers()

This fixes an issue where swapbuffers() did not swap the
"back buffer" to the "front buffer" if we had single-buffered swap
behavior.
This was an issue on Windows 7 with an NVIDIA graphics card that
by default was using single-buffered mode.

Change-Id: If53f54146e4633305be3ad5158565752b6516b59
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>

src/gui/kernel/qopenglcontext.cpp

@@ -567,7 +567,8 @@ void QOpenGLContext::swapBuffers(QSurface *surface)
     if (framebufferBinding != platformFramebuffer)
         qWarning() << "QOpenGLContext::swapBuffers() called with non-default framebuffer object bound";
 #endif
-
+    if (surface->format().swapBehavior() == QSurfaceFormat::SingleBuffer)
+        glFlush();
     d->platformGLContext->swapBuffers(surfaceHandle);
 }