Compositor testing methodology

[myselfhimself]

- ongoing D6334 Blender patch on compositor testing (node setup with expected output vs. real output) - A GSoC 2020 is ongoing on regression testing here, where the blender-addon-tester which we use has pointed out as a comment

[myselfhimself]

This depends on #10 to be implemented first.

[myselfhimself]

gmic-py I/O non-regression UI-less is being added in https://github.com/myselfhimself/gmic-py/issues/36 , shall be out for gmic-py 2.9.0 end of this month. When this is solid enough, Blender non-regression will be easier to implement.