Compute the barycentric coordinates (A,B,C) of the Point relative to the specified TriangleID of the TargetMesh. The properties of barycentric coordinates are such that A,B,C are all positive, A+B+C=1.0, and A*Vertex1 + B*Vertex2 + C*Vertex3 = Point. So, the barycentric coordinates can be used to smoothly interpolate/blend any other per-triangle-vertex quantities. The Point must lie in the plane of the Triangle, otherwise the coordinates are somewhat meaningless (but clamped to 0-1 range to avoid catastrophic errors) The Positions of the Triangle Vertices are also returned for convenience (similar to GetTrianglePositions)