Disable threaded FBO canvas drawing when not supported.

Change-Id: I1e8300c61ba31a5400fc43f85a9c39d2245b4518
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Reviewed-by: Alan Alpert <aalpert@blackberry.com>