reduce opengl2 module dependencies
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.
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