Expose QOpenGL2PaintEngineExPrivate pvmMatrix

The pvmMatrix matrix is used to render 2D primitives when rendering to an OpenGL surface.  To match the rendering pipeline of Qt when overloading QGraphicsItem painter, this matrix is very useful. 

In my example, I am rendering an image with a complex shader, but want the benefits of a QGraphicsItem that is managed by a QGraphicsView.