Support dynamically switching mesh type at runtime

With this change you can now update the mesh type while the game is loaded:

    game.meshType = 'surfaceMesh'
    game.meshType = 'wireMesh'

New chunks will have the new mesh type and old meshes will be properly cleaned up:

- Remove available mesh from scene, instead of current mesh (fixes overlap)
- Dispose any mesh, instead of current mesh (fixes crash)
- Add missing delete for wireMesh
Add showAllChunks() to re-render all chunks

Useful for refreshing after changing rendering settings.
Merge pull request #93 from deathcap/changemesh

Support dynamically changing mesh type at runtime