Triangles owned by a part may have some vertices owned by another part.
This is unavoidable for skin animated models because triangles can be made
from vertices owned by different parts. In Seamless3d the rule is a parts
owns a triangle if it owns a vertex in the triangle and it is the part
indexed the lowest in the part list.
The index order of the part list is determined by where the part is
located in the scene tree as shown in the following example:
Therefor if for example a triangle had a vertex owned by the shoulder (indexed at 11) and a vertex owned by neck0 (indexed at 4) and a vertex owned by the thorax (indexed at 3) the thorax would own the triangle because it is indexed the lowest in order. (the first in the list wins)