This makes it a lot more convenient as we then have direct access to the
GL context, createTexture(), invalidation signal and more.

Change-Id: I114514d1379c4d0efcc35c512fa2b485da3438b0
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>
Reviewed-by: Kim M. Kalland <kim.kalland@nokia.com>