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.
starting from the objects of lowest ObjectID.
(convex hulls are recoverable as well as the indexed triangle sets
inside the TriangleMeshes or the triangle connectivity information).
Now the top most snapshot (the temp one taken before Undo jump) will
never be released.
Fixed one bug of many in admesh library, where the face connectivity
was broken when removing a degenerate face. Likely there are some
similar issues left to be solved.
Placed a workaround to always recalculate face connectivity before slicing.
