Working small-to-normal support merging

Fixed fatal bug with anchors for mini supports

Make the optimization cleaner in support generatior

Much better widening behaviour

Add an optimizer interface and the NLopt implementation into libslic3r

New optimizer based only on nlopt C interfase
Fix build and tests

src/libslic3r/SLA/SupportTreeMesher.hpp

@@ -94,6 +94,24 @@ inline Contour3D get_mesh(const Bridge &br, size_t steps)
     return mesh;
 }
 
+inline Contour3D get_mesh(const DiffBridge &br, size_t steps)
+{
+    double h = br.get_length();
+    Contour3D mesh = halfcone(h, br.r, br.end_r, Vec3d::Zero(), steps);
+
+    using Quaternion = Eigen::Quaternion<double>;
+
+    // We rotate the head to the specified direction. The head's pointing
+    // side is facing upwards so this means that it would hold a support
+    // point with a normal pointing straight down. This is the reason of
+    // the -1 z coordinate
+    auto quatern = Quaternion::FromTwoVectors(Vec3d{0, 0, 1}, br.get_dir());
+
+    for(auto& p : mesh.points) p = quatern * p + br.startp;
+
+    return mesh;
+}
+
 }} // namespace Slic3r::sla
 
 #endif // SUPPORTTREEMESHER_HPP