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/IndexedMesh.hpp

@@ -87,7 +87,7 @@ public:
         inline Vec3d position() const { return m_source + m_dir * m_t; }
         inline int face() const { return m_face_id; }
         inline bool is_valid() const { return m_mesh != nullptr; }
-        inline bool is_hit() const { return !std::isinf(m_t); }
+        inline bool is_hit() const { return m_face_id >= 0 && !std::isinf(m_t); }
 
         inline const Vec3d& normal() const {
             assert(is_valid());