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).
cdabf7e353