Fix QPixmap and OpenGL threading warnings when painting

If the QPA supports threaded pixmaps, don't print out a message warning
the user about painting a pixmap in the non-main thread.
Likewise, if the QPA supports threaded OpenGL, don't print out a warning
when the user is about to paint on OpenGL from a non-main thread.

Change-Id: I5b4928ee49b1d6c1b638b036fade12dc794258df
Reviewed-by: Albert Astals Cid <albert.astals@canonical.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>