Fixed crash of jsonmodels example on OS X.

Added QTCANVAS3D_gl_state_dump extension for dumping the current GL state for debugging purposes and added example use to jsonmodels example.
Fixed documentation issues and removed invalid comments related to render thread.

Change-Id: Ib4a81370bf2673a5bfb412038148646638f723a9
Reviewed-by: Tomi Korpipää <tomi.korpipaa@digia.com>
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>