Made sure the C++ fillers are instantiated at the worker threads,
where there are being released.
Extended the FillRectilinear2 to calculate the contour / line intersection
with exact arithmetics, improved robustness and added error handling
and error reporting, if the contours to be filled are not correct.
Added a safe variant of offset(const Slic3r::ExPolygon...), which offsets each loop separately.
New functions "remove_sticks" to remove zero area parts of polygons.
New functions "remove_small" and "remove_degenerate" for polygon clean up.
Extended the C++ supports, those are not finalized yet though.
these regions are grown to anchor the bridge lines to the bottom surface.
The grown regions may overlap. In that case the regions are now merged
before the bridging direction is calculated for the merged region.
