Fixes of the offset curves from Voronoi diagram.

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.

src/libslic3r/Geometry.cpp

@@ -471,7 +471,7 @@ Pointfs arrange(size_t num_parts, const Vec2d &part_size, coordf_t gap, const Bo
     size_t cellw = size_t(floor((bed_bbox.size()(0) + gap) / cell_size(0)));
     size_t cellh = size_t(floor((bed_bbox.size()(1) + gap) / cell_size(1)));
     if (num_parts > cellw * cellh)
-        throw std::invalid_argument(PRINTF_ZU " parts won't fit in your print area!\n", num_parts);
+        throw std::invalid_argument("%zu parts won't fit in your print area!\n", num_parts);
     
     // Get a bounding box of cellw x cellh cells, centered at the center of the bed.
     Vec2d       cells_size(cellw * cell_size(0) - gap, cellh * cell_size(1) - gap);