Render all cached QGraphicsItems first, then render the rest of the scene.

When using a hardware acceleration like OpenGL or OpenVG, switching render targets is very expensive. If the QPixmap backend is rendered into using one of these APIs, the cost of switching rendering to the QPixmap and back again is very high. It would be beneficial if QGraphicsView could collect up all the QGraphicsItems which will be re-directed to QPixmaps and render them all in one pass before switching to the actual viewport to do the render. This could potentially halve the number of render target changes. However, if QPixmaps are being rendered into using the raster paint engine (as they currently are) then the performance hit is negligible.