Use threaded renderer even without BufferQueueingOpenGL.

Use threaded renderer even without BufferQueueingOpenGL.

Without threaded rendering QML 2 often ends up running at half the
screen refresh rate on X11. Not having BufferQueuingOpenGL shouldn't
prevent us from using threaded rendering, since the issue of four
animating windows running at 1/4th of the screen refresh rate will
happen regardless of whether rendering happens on the main thread or in
a separate rendering thread.

Change-Id: I0edc818e6d762fa1faf13e7f2f47dfda132b6fdf
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>