updates to eliminate 2d artifacts from difference operations

[x0pherl]

The "difference" operation in openscad creates artifacts on render if the bit you're subtracting doesn't exceed the borders of the bit you are starting with -- it creates a sort of shimmery plane in preview that's pretty annoying (although it doesn't impact the fully rendered part)

I've added a simple fix here that adds 0.01mm of extra size to the parts being subtracted so that rendering artifact goes away.