Adds support for raycasting with morphs

This isn't insanely fast, but it stops the bug described [here](https://stackoverflow.com/questions/13723473/three-js-ray-intersection-when-using-morphtargets-not-working), where the `RayCaster` assumes all meshes are in their unmorphed form.

Tested on a mesh with 10000 vertice, runs at a reasonable speed. The code path should only be taken if morphs are enabled anyway.