Working contour offsetting,
skeleton_edges_rough() to detect "important" skeleton edges.
Radius of an inscribed circle along the "important" skeleton edges
changes slowly, therefore these "important" skeleton edges signify
oblong regions possibly needing a gap fill.
The offset curve extractor is already quite usable,
though singular cases are still not covered yet
when the offset curve intersects or nearly intersects
a Voronoi vertex.
Removal of the PRINTF_ZU "%zu" Visual Studio printf compatibility macro.
Fixes of a contours self intersection test for collinear segments.
SVG exporter now exports white background, so that the GNOME Eye viewer is usable.
Now the algorithm is very different from the OpenVoronoi implementation
and hopefully it is now correct (save numerical issues, which will be
a big PITA).
