abstracted construction logic of morphTarget to `updateMorphTargets`

Now instead of running morphTarget logic only during construction of a `THREE.Mesh` you can call it on the fly. For convenience it is invoked during construction as well.

This allows for more dynamic creation of morph targets. e.g:

1. Make a mesh
2. Update the geometries morph targets
3. Update the materials boolean property `morphTargets` to `true`
4. Call `mesh.updateMorphTargets()`