Parameters for surfaceIndexFunction are redundant

[dbisdorf]

The first parameter to the surfaceIndexFunction callback is the surface type (wall/ceiling/floor); however, this is somewhat redundant. If the face parameter is 1, 2, 3, or 4, we must be asking for a wall; if the parameter is 0, we're asking for a ceiling or floor.

We could eliminate the first parameter (surface type) and simply pass 5 or 6 through the face parameter if we want a ceiling or floor.

[dbisdorf]

I've updated the library, the example game, and the README for this issue.