11/08/93: add a delay to inputserver loop to reduce the usage of CPU power
07/09/93: add linear interpolation in RT_Image::get_icolor
24/11/93: add more lights:
	DirectionalLight
	PositionalLight
24/11/93: add smooth shadows
24/11/93: dynamic loading 
24/11/93: interuptable raytracing
24/11/93: RT_PixmapDisplay::get_pixel for each Port (PHIGS, MS Windows)
24/11/93: extend Tcl class system: private methods, overloading, ...
24/11/93: CSG
24/11/93: Brep
24/11/93: combined object Light/Prim: move a top object together with a light
25/11/93: Raytracer special effects (fog,...)
25/11/93: more concrete texture and bump mapping classes, displacement mapping
25/11/93: animation functionality
25/11/93: more image classes (GIF, TIFF, ...)
25/11/93: time based input devices (RT_Timer)
15/04/94: tuning of isA, use enums for groups of types to speed up type checking
15/04/94: add sweep modelling