This commit fixes several issues:
* island_store->items_to_islands_num was reset each time we added a new island, this is stupid! Harmless too, though, afaikt.
* partial verts bvhtree (with several islands) was hugely over-allocated...
* we would 'leak' in neighbor islands when geometry itself was contiguous.
* best_nor_dot was used incorrectly, leading to smaller weights for better matching normal!
All those fixes are related to T44522 (through personal communications with reporter).
Caused by own commit that changed island detection code. In the case of
modifiers we don't want to take winding information into account, but
left the code since there are use cases (like painting) which could use
this.
Issue here is simple and has been fixed in other places such as
texpainting: Basically if face has different winding, do not calculate
it as adjucent to the other face, even if UV is identical.
This allows us to stack islands of symmetrical closed meshes on top of
one another and still be able to select the two identical island halfs
(provided the normals are correct of course).
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!
