Don't use the private QQuickDefaultClipNode and QSGRectangleNode

Use QSGSimpleRectNode and a custom QSGClipNode instead, which provide
the same funtionalities without using private API.

Change-Id: Ia62ddc9cf86240dbf85f69c088599b7ca8792e7c
Reviewed-by: Andras Becsi <andras.becsi@digia.com>