; this is really nothing at the moment, but the intention is to allow proper shadowing as follows:

; 1. when building the complete 3d structure in the track generator, allow an extra stream, the shadow stream,
;    of exactly the same size as the buildings stream.
; 2. taking each polygon of the object in turn, project its points onto the ground along the sun direction, and
;    create a new object, which can then be used for the shadows.
; note that only polygons which are facing the sun need to be used (test by assuming viewpoint is sun - ie take the 
; object zero point, and subtract $10000 sun vectors from it to give a viewpoint)
; because we assume a distant source, giving parallel rays, we can project without using the viewpoint.
; However the shape projected could be a problem, as overlapping will cause non-constant shadows, which would be silly.
; this is no problem for a cube - each faces shadow should match perfectly with all others - but in the case of a building which
; is non-convex, there will be problems.
; The obvious answer is to perform clipping of each new shadow polygon against the old shadow polygons, although perhaps a better
; solution would be to change the drawing routine to allow concave polygons.  Then the solution is to combine the polygons into 
; one, which would require the same process as for the clipping, but would cut the drawing time slightly.


