Computes the barycentric coordinates of the
given point p.
The point p must lie on the same plane as the triangle this; if the point is not coplanar,
the result of this function is undefined.
If we place the origin in the coordinates of the triangle's A point, the barycentric coordinates are `u`, which is on the AC vector; and `v` which is on the AB vector:
The returned Vec2 contains the following values, in order:
| this |
a Triangle |
| p |
a Point3D |
| res |
return location for the vector with the barycentric coordinates |
|
`true` if the barycentric coordinates are valid |