Reduce opengl2 dependencies in two ways
first move a class that renderingcore depends on from
filterExtraction to FiltersGeneral. FiltersExtraction pulls
in a lot of other modules where FiltersGeneral does not.
FiltersExtraction already depends on filtersGeneral so it
should be backwards compatible.

The second change is a new feature to allow a back implementation
to be required or not. In this case gl2ps was originally required
by OpenGl2 now it is optional. This required adding an option to
the module code to specify when a backend implementation is required.