Csg import tests

Add python script to validate exported CSG files against the original SCAD file. This mainly ensures that we can parse our own files again and they produce the same output as when rendering the SCAD file.

* Use all test cases from CGALPNGTEST_FILES
* Call OpenSCAD once to parse *.scad and export to *.csg
* Call OpenSCAD a second time to render the *.csg file output to PNG
* Validate against the output images of the original *.scad test cases
* Two test cases disabled (csgpngtest_primitive-inf-tests csgpngtest_transform-nan-inf-tests), those need a decision how to handle inf/nan in export

Tasks
- [ ] Check if the python script works in the new Windows test case package