Move QSGRenderer::updateStencilClip down to QSGBatchRenderer

This convenience method is currently only used by the batch renderer.
Moving it allows removing the QOpenGLFunction inheritance of
QSGRenderer and unbinding it slightly from the rendering implementation.

Change-Id: I4322952f843de8d950ced32885feee8d6c4a2730
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>