Implement tiled drawing in drawPixmap for the VG paint engine

QVGPaintEngine::drawPixmap() will often fail with large pixmaps. This can happen due to lack of sufficient GPU memory or hitting size limitations (e.g. 2048x2048 on Symbian). There is a fall back to vgWritePixels which may help in certain simple cases (no alpha channel, simple matrix), but another fall back to a slow but reliable tiled drawing (create a small VGImage, upload, draw, upload, draw, ...) would be beneficial.