Fix issue sharing OpenGL context from render thread on OS X

Fix issue sharing OpenGL context from render thread on OS X

On OS X when running in QtQuick 2 examples in debug mode we fail an
assert because we try to set a dynamic property using
QObject::setProperty from the render thread, while the object exists in
the main thread.  Now we call setProperty from the correct thread.

Change-Id: I3f26ead0f68fadcded472bf5c9014a4025f0a03e
Reviewed-by: Christian Stromme <christian.stromme@digia.com>
Reviewed-by: Jason Barron <jason.barron@digia.com>