Use Vector3's setFromMatrixPosition if available

Vector3's .getPositionFromMatrix() has been renamed to .setFromMatrixPosition() in r64 (specifically in [this commit](https://github.com/mrdoob/three.js/commit/bb31515d6bae3d7ba10541f5ccaa490c72c1ac6e)).

Adding a conditional that uses the new name if it is available, and the old if not.