Reworked polygon sampling for SLA auto support generation.

Conditional compilation of an igl winding number tree for SLA support generator, as it is not used as of now and initialization of the tree is expensive.
Fixed issue with passing the new SLA point definition to the back end and back to the UI.

src/libslic3r/SLA/SLACommon.hpp

@@ -3,6 +3,7 @@
 
 #include <Eigen/Geometry>
 
+// #define SLIC3R_SLA_NEEDS_WINDTREE
 
 namespace Slic3r {
     
@@ -11,6 +12,7 @@ typedef Eigen::Matrix<float, 3, 1, Eigen::DontAlign> Vec3f;
 typedef Eigen::Matrix<double, 3, 1, Eigen::DontAlign> Vec3d;
 
 class TriangleMesh;
+
 namespace sla {
     
 struct SupportPoint {
@@ -115,11 +117,13 @@ public:
         int F_idx() const { return m_fidx; }
     };
 
+#ifdef SLIC3R_SLA_NEEDS_WINDTREE
     // The signed distance from a point to the mesh. Outputs the distance,
     // the index of the triangle and the closest point in mesh coordinate space.
     si_result signed_distance(const Vec3d& p) const;
 
     bool inside(const Vec3d& p) const;
+#endif /* SLIC3R_SLA_NEEDS_WINDTREE */
 };