This is the (big!) core of mesh transfer data, it defines a set of structures
to represent a mapping of mesh elements (verts, edges, polys of loops) between
two arbitrary meshes, and code to compute such mappings.
No similarity is required between source and destination meshes (though results
when using complete different meshes are rather unlikely to be useful!).
This code is not bound to data transfer, it is defined to be as generic as possible,
and easy to reuse or extend as needs arise.
Several methods of mapping generation are defined for each element type,
we probably will have to adjust that in future (remove useless ones, add
new ones...).
For loops, you can also define islands (for UVs e.g.) so that loops of a same
destination polygon do not 'spread' across several source islands.
Heavily reviewed and enhanced by Campbell, thanks a lot!
* Consider non-manifold edges as sharp, as in split normals handling.
* Consider edges from sharp polys as sharp!!!
* Fix returned number of groups (was off-by-one for non-bitflags grouping, could also
be wrong in case of id overflow).
Note about using sharp polys too to define groups: Only current use of this function
(Obj exporter) does not need that, because it does its own check for sharp faces.
However, we might reuse that func in other places in future (e.g. in custom split
normals area), so better to get a consistent behavior!
