Added possibility to test fore/background rendering target and
added two tests that automatically switch canvases rapidly.

Change-Id: I8de2d54aa36018ce384a9b3c92b3d38287fc8c71
Reviewed-by: Tomi Korpipää <tomi.korpipaa@theqtcompany.com>
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>