Refactoring of adaptive cubic / support cubic:

1) Octree is built directly from the triangle mesh by checking
   overlap of a triangle with an octree cell. This shall produce
   a tighter octree with less dense cells.
2) The same method is used for both the adaptive / support cubic infill,
   where for the support cubic infill the non-overhang triangles are
   ignored.
The AABB tree is no more used.
3) Optimized extraction of continuous infill lines in O(1) instead of O(n^2)

src/libslic3r/Fill/Fill.cpp

@@ -345,8 +345,7 @@ void Layer::make_fills(FillAdaptive_Internal::Octree* adaptive_fill_octree, Fill
         f->layer_id = this->id();
         f->z 		= this->print_z;
         f->angle 	= surface_fill.params.angle;
-        f->adapt_fill_octree = adaptive_fill_octree;
-        f->support_fill_octree = support_fill_octree;
+        f->adapt_fill_octree = (surface_fill.params.pattern == ipSupportCubic) ? support_fill_octree : adaptive_fill_octree;
 
         // calculate flow spacing for infill pattern generation
         bool using_internal_flow = ! surface_fill.surface.is_solid() && ! surface_fill.params.flow.bridge;