Leave the depth mask on after rendering.

Try to be compatible with raw GL when possible.

Change-Id: Id6e17da2d8c134362a5428de970cd5e562f57143
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Introducing QQuickItem::sceneGraphInvalidated/sceneGraphInitialized

[ChangeLog][QtQuick][QQuickItem] Added signals sceneGraphInitialized and sceneGraphInvalidated

Change-Id: Idaea88bc743f0637d093cf1ba7ac4f78acd7e6ad
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>