Rename --render=cgal to --render=force to force-convert to the current backend-specific geometry

[kintel]

--render=cgal was really meant for converting to the internal format of the current geometry engine to utilize any of its built-in mesh validation and repair tools. When using another geometry backend, we're changing this to --render=force to force conversion into the current backend-specific geometry.

--render=cgal is still useable for backwards compatibility, but not sure for how much longer we need to support it.

Also added some opportunistic refactoring.

[jordanbrown0]

Yes, you should rename it. "--render=cgal" should use CGAL, or be an error if CGAL is not available. Otherwise it's confusing.

[kintel]

--render=cgal is also slightly confusing as both the regular backend and fast-csg are CGAL.

I believe the initial purpose of this flag was:

• --render: Force concrete geometry to be rendered. If the .scad file only contains concrete geometry, e.g. only consists of a primitive, an import(), a basic linear extrude etc., it will use that geometry as is, i.e. a PolySet, otherwise, the boolean engine will be invoked and produce whatever geometry that backend provides (Nef3 polyhedrons for the release build, but could be ManifoldGeometry).
• --render=cgal: If the resulting geometry from evaluating the file was a PolySet, convert it to CGAL Nef3 polyhedrons, then back to PolySet. This ensures that the geometry passes all checks for valid polyhedrons in CGAL, or allows issues to be triggered instantly.

Ideally, we should rename --render=cgal to something more generic which doesn't involve naming the backend.

If we do ship with multiple backends (either for a release build or as experimental), having a flag to select backend would make sense, but that could be separate from --render=.

[kintel]

Update: Renamed --render=cgal to --render=force