ex05: display all colors from the ray-traced buffer again
ex05: fix a typo in a var name
ex05: make some functions static
ex05: remove doubled version of intersect_triangle again (is in Scene.cpp already)
ex05: minor style stuff related to GetIntersectionPos()
ex05: only convert toPPM values to 0 - 255 range (from 0.0 - 1.0)
ex05: introduce BACKGROUND_COLOR constant
ex05: max intensity per light depends on number of lights
ex05: use correct sign for light direction vector