Avoid deep copy when updating BitmapTextureGL

Since we use a ImageBuffer to paint into a BitmapTextureGL, and
ImageBuffer's always have an active painter, the convertion from QPixmap
to QImage causes a deep copy of the content before it is uploaded to
OpenGL. This copy is unnecessary and can be avoided by temporarily
redirecting the QPaintDevice.

Change-Id: I0106c0f6dfb9cc3fbbee943411fc8f6853df4f98
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>