Better fill-gaps handling, fix for #1803

When calculating infill areas, gaps are filled first with medial_axis, resulting polylines are offset to flow width and subtracted from infill area. This ensures that gaps are not filled twice, but allows infill in areas where medial_axis did not find solution.
It also fixes #1803 - expanded polyline does not contain problematic artifacts. 

Could use some style and efficiency improvement - only fgaps needs to be subtracted from infill and my perl is even worse than my English ...