Drawing a QPixmap to QImage causes a deep copy on Symbian

When drawing a QPixmap to QImage, raster paint engine needs to call QPixmap::toImage() to get the raster data.
On Symbian on OpenVG and OpenGL graphics systems this causes a deep copy because QPixmap is backedup by CFbsBitmap.
Raster engine should use QPixmapData::toVolatileImage() on Symbian to avoid deep copy.