Add basic fog

- Very basic fog, based on a fog color and near/far parameters.
- Can toggle fog / adjust parameters in javascript (passed as uniforms)
- Uses the fragment shader of the accessory buildings to mix the color
  of the buildings with the fog color

TODO: Make fog work with the skydome? Though we might remove the
skydome once we have the "superbuildings", so it might not be as
necessary.

TODO: Make fog dependent on the "level number" of the world.