Make `CesiumTileExcluder` material responsive

[j9liu]

I think that 13_CesiumTileExcluder would be a more effective sample if the material actually accounted for the box size and position. (Rotation would be a cool plus, but perhaps is ambitious).

Currently, it discards fragments outside of a fixed world-position area. So if you move the trigger box (and therefore move the exclusion zone), you get this:

It's not a bug; the material is simply set up that way. But it would be more intuitive and powerful to make it account for changes in the box.