1) New methods PrintObject::num_regions() and ::has_region() to make

the code more readable and to highlight where PrintObject::region_volumes
   are actually set and consumed.
2) Replaced Slic3r::clamp() with std::clamp(). They differ in the order
   of their parameters, thus hopefully no new bugs were introduced.
3) Some refactoring of MultiMaterialSegmentation for efficiency.

src/libslic3r/Fill/FillGyroid.cpp

@@ -53,7 +53,7 @@ static inline Polyline make_wave(
     polyline.points.reserve(points.size());
     for (auto& point : points) {
         point(1) += offset;
-        point(1) = clamp(0., height, double(point(1)));
+        point(1) = std::clamp(double(point.y()), 0., height);
         if (vertical)
             std::swap(point(0), point(1));
         polyline.points.emplace_back((point * scaleFactor).cast<coord_t>());