Add missing delete for wireMesh
Remove available mesh type from scene, instead of current mesh

Allows you to dynamically change the mesh type at runtime:

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

then explore new chunks, and revisit old (unloaded) chunks -- the
new meshes will be displayed correctly, instead of overlapping the
older mesh type. With this change and c845d3fd, meshType is now only
used for determining the type when _creating_ new meshes.
Add showAllChunks() to re-render all chunks

Useful for refreshing after changing rendering settings.