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/Fill3DHoneycomb.cpp

@@ -55,8 +55,8 @@ static std::vector<coordf_t> perpendPoints(const coordf_t offset, const size_t b
 static inline void trim(Pointfs &pts, coordf_t minX, coordf_t minY, coordf_t maxX, coordf_t maxY)
 {
     for (Vec2d &pt : pts) {
-        pt(0) = clamp(minX, maxX, pt(0));
-        pt(1) = clamp(minY, maxY, pt(1));
+        pt.x() = std::clamp(pt.x(), minX, maxX);
+        pt.y() = std::clamp(pt.y(), minY, maxY);
     }
 }
 

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>());