ENH: only draw infill in normal nodes of hybrid support

1. Only draw infill in normal nodes of hybrid support.
  Previously when hybrid support was selected, all nodes would have
  infill including the circle nodes which don't need infill. Now we draw
  infill only when there are normal nodes.
2. Move remove_bridge after small overhang removal because the trimmed
  bridge may be detected as small overhangs.
3. Only split circle nodes in draw_circles to prevent floating circles.
4. Fix the issue where tree support infills may not be aligned,
   which is caused by the changing spacing of filler.
5. Fix a bug where lightning infill in tree support may crash.
   Note: it is still not prepared to set lightning infill as default,
   as for some models the generation time of lightning infill is still
   too long.

Change-Id: I556e5097041d09afae1e2957d4dc9914d4610149
(cherry picked from commit 4c0849a81b60cde0c3b8ca54f03f63ea7c62dcd9)

src/libslic3r/Layer.hpp

@@ -269,6 +269,7 @@ protected:
         ExPolygon *area;
         int        type;
         coordf_t   dist_to_top; // mm dist to top
+        bool need_infill = false;
         AreaGroup(ExPolygon *a, int t, coordf_t d) : area(a), type(t), dist_to_top(d) {}
     };
     enum OverhangType { Detected = 0, Enforced };