This adds a concept of a rendering backend to the module API, with
an OpenGL and OpenGL2 backend. They are mutually exclusive and one
or the other should be selected (or None in the case when rendering
is not requested). This needs a little more work as the default
backend enables rendering when the group is disabled at this point.

Change-Id: I98d660e063ef58d52698e08ab06dd45601bea9cd