surface: Separated SurfaceBuilding.cpp's graphics under draw directory.
surface: Add Airport model, but not dynamics shape yet.

SurfaceBuilding class has had a bug that distinct model for each
instance of it could not be used.  That bug is fixed in this commit,
too.
surface: Make SurfaceBuilding to accept landOffset from scripts.

The landOffset parameter controls relative position of a
SurfaceBuilding's model origin.
surface: Improved Airport model's textures.