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/GCodeSender.cpp

@@ -393,7 +393,7 @@ GCodeSender::on_read(const boost::system::error_code& error,
                 }
                 this->send();
             } else {
-                printf("Cannot resend " PRINTF_ZU " (oldest we have is " PRINTF_ZU ")\n", toresend, this->sent - this->last_sent.size());
+                printf("Cannot resend %zu (oldest we have is %zu)\n", toresend, this->sent - this->last_sent.size());
             }
         } else if (boost::starts_with(line, "wait")) {
             // ignore