With a sun lamp shining exact perpendicular to a face, the result was
undefined (black or full lit). Was caused by using acos() without checking
the input range. Float calculus can result in small under or overflows...
therefore: always use the arithb.c function saacos() !!!
Found more acos() usage in rendercore (in blinn and arealight too), removed
all of them. Might solve the white or black dots people were complaining
about?
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