Bugfix: custom seam did not work for triangles that were exactly vertical

The projection algorithm was originally made for custom supports, so vertical triangles
were not interesting. They are for seam. Instead of rewriting the algorithm and risking
more bugs, the edge case is detected and the triangle is tilted slightly.

src/libslic3r/PrintObject.cpp

@@ -2845,6 +2845,11 @@ void PrintObject::project_and_append_custom_facets(
             if (! seam && tr_det_sign * z_comp > 0.)
                 continue;
 
+            // The algorithm does not process vertical triangles, but it should for seam.
+            // In that case, tilt the triangle a bit so the projection does not degenerate.
+            if (seam && z_comp == 0.f)
+                facet[0].x() += float(EPSILON);
+
             // Sort the three vertices according to z-coordinate.
             std::sort(facet.begin(), facet.end(),
                       [](const Vec3f& pt1, const Vec3f&pt2) {