Now generate the object factory files in a function

This moves the logic for object factory override generation to a
central function, and removes lots of copy/paste code in our CMake.
Also removed/retired the OpenGL2 object factory stomping variation
as it is no longer needed and could produce unwanted side effects.

Change-Id: I4a5ee2db76f73d57eded1d9a9240cefc4918cd74