TriangleMesh newly only holds indexed_triangle_set and
TriangleMeshStats. TriangleMeshStats contains an excerpt of stl_stats.
TriangleMeshStats are updated when initializing with indexed_triangle_set.
Admesh triangle mesh fixing is newly only used when loading an STL.
AMF / 3MF / OBJ file formats are already indexed triangle sets, thus
they are no more converted to admesh stl_file format, nor fixed
through admesh repair machinery. When importing AMF / 3MF / OBJ files,
volume is calculated and if negative, all faces are flipped. Also
a bounding box and number of open edges is calculated.
Implemented its_number_of_patches(), its_num_open_edges()
Optimized its_split(), its_is_splittable() using a visitor pattern.
Reworked QHull integration into TriangleMesh:
1) Face normals were not right.
2) Indexed triangle set is newly emitted instead of duplicating
vertices for each face.
Fixed cut_mesh(): Orient the triangulated faces correctly.
Fix compilation on Windows
Fix array subscript out of range error in MarchingSquares
Fix normals of mesh constructed from slices
Improve performance of mesh construction from slices
