Refactor functions to encapsulate private cache variables

[gkjohnson]

A lot of functions depend on cached THREE objects:

const v1 = new Vector3();
function doSomething( triangle ) {

  v1.copy( triangle.a );
  // ... do stuff ...

}

which runs a risk of functions overwriting other functions variables mid-use, etc. It might be best to refactor to use the pattern that THREE.js uses:

const doSomething = ( function() {

  const v1 = new Vector3();
  return function doSomething ( triangle ) {
    // ...
  }

} )();

Or for class members:

MeshBVHNode.prototype.doSomething = ( function() {

  const v1 = new Vector3();
  return function doSomething ( triangle ) {
    // ...
  }

} )();

Update: See MeshBVH.js for the functions that need to be updated.