Modify the auxiliary spine to be a true helix, resulting in a much
smoother linear extrusion when a twist angle is applied. This also
corrects a bug in the scaling during linear extrusion, where non-uniform
scaling was not handled correctly.
The original implementation of the surface() function used a simple
B-spline representation for the surface, which generated degenerate
surface with several of OpenSCAD's demo input files. This commit
modifies the algorithm to generate a discrete surface identical to the
one generated within OpenSCAD itself. It also adds several units tests
to identify future regressions.
Note that PNG input is not yet supported for the surface() function.
The OpenSCAD module did not have any unit tests. This commit adds the
basic framework (and two very simple tests). The framework is modeled
after the unit tests in the FEM module.
