Highlight negative regions when rendering using Manifold

[kintel]

When using the (default) CGAL Nef polyhedron backend, F6 rendering results in "negative surfaces" (cutouts) being rendering in a different color. We should implement the same for the Manifold backend.

Example for a difference between a sphere and a cube:

A similar feature is demoed on https://manifoldcad.org/#Tetrahedron%20Puzzle

A related opportunity with Manifold is to also support preserving color() in F6 rendering. A prototype has shown that this is doable in OpenSCAD. Keep that in mind when solving this ticket by making the rendering backend flexible enough to handle multiple colors.

[gsohler]

kintel, see this one

https://github.com/gsohler/openscad/blob/color/src/geometry/manifold/ManifoldGeometry.cc#L114-L117
triangle points runIndex[0-1] -> yellow
triangle points runIndex[2-3] -> green

[gsohler]

BTW my code is wrong
for(int i=0;i<mesh.runIndex.size()-1;i++) {

should be:

for(int i=0;i<mesh.runIndex.size();i+=2) {