ENH: use print volume to clip tree support

So tree supports won't go outside the bed.

Known issue:
1. moving won't trigger support re-calculating, so if you want to
  clip the supports in a different way after moving, you need to change
  the support settings (eg change threshold angle to 31 degrees).
2. clipping with the occlusion region is not complete, and an error
  message of "outside toolpath" will still be popped because we use
  convex hull to detection confliction.

Jira: STUDIO-2036

Change-Id: I643b14618eb18ffa9825072c44f677e51b0ff937
(cherry picked from commit a6217824dc0f490027e16f80f810d176dec6004b)

src/libslic3r/TreeSupport.hpp

@@ -125,11 +125,6 @@ private:
      */
     const ExPolygons& calculate_avoidance(const RadiusLayerPair& key) const;
 
-    /*!
-     * \brief Polygons representing the limits of the printable area of the
-     * machine
-     */
-    ExPolygon m_machine_border;
 
 public:
     bool is_slim = false;
@@ -401,6 +396,12 @@ private:
     bool  with_infill                        = false;
 
 
+    /*!
+     * \brief Polygons representing the limits of the printable area of the
+     * machine
+     */
+    ExPolygon m_machine_border;
+
     /*!
      * \brief Draws circles around each node of the tree into the final support.
      *