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/ElephantFootCompensation.cpp

@@ -270,7 +270,7 @@ std::vector<float> contour_distance2(const EdgeGrid::Grid &grid, const size_t id
 				    const Vec2d   v  = (segment.second - segment.first).cast<double>();
 				    const Vec2d   va = (this->point - segment.first).cast<double>();
 				    const double  l2 = v.squaredNorm();  // avoid a sqrt
-				    const double  t  = (l2 == 0.0) ? 0. : clamp(0., 1., va.dot(v) / l2);
+				    const double  t  = (l2 == 0.0) ? 0. : std::clamp(va.dot(v) / l2, 0., 1.);
 				    // Closest point from this->point to the segment.
 				    const Vec2d   foot = segment.first.cast<double>() + t * v;
 					const Vec2d   bisector = foot - this->point.cast<double>();