1. QQuickPixmap now only store textures instead of QImage data, however
context2d still need to access the QImage in some places, so cache the
loaded images to avoid the expensive GL readback operations.
2. Use texture directly if the render target is FBO.

Change-Id: I6228011e5698fa00f2e3420a3a4a305995b8a238
Reviewed-by: Yunqiao Yin <charles.yin@nokia.com>