RayShade and spot lights.

[Roger H. <rog...@mi...>]

I have improved the spot light handling code in Quesa's RayShade  
renderer. Would anyone like to test and check in the changes? Before  
they did not seem to work so I lifted some code I wrote for the  
Microspot Renderer and it seems to work quite well now, though it may  
need optimisation some time.

Roger Holmes.