minkowskiDifference alg bug #60

See this commit:
JansenSmith/ProgrammaticFrameBacklighting@f91c4c0

Expected:

made with this toolOffset:

trench_frame_front = trench_frame_front.difference(cclamp.hull().toolOffset(0.91))

Current Minkowski result:

made with this minkowskiDifference:
trench_frame_front = trench_frame_front.minkowskiDifference(cclamp.hull(), 0.91)