title: Flip normals in three.js on sphere
body: <p>I have been searching around and haven't found any really good answer to my question yet..
  The thing is that I have this sphere.. just a basic sphere, and I want to flip the normals so
  the sphere gets the sort of "hollow/carved effect" and then plater on apply my textures to the "inside" of the sphere. any ideas of how to flip the normals?</p>
  
  <p>Also.. if its not possible to do this in three.js.. would it be possible to import a model where the normals are already flipped and get the effect I'm looking for?</p>