ENH: improve sharp detection for small regions

1. improve sharp detection for small regions.
   When the region is around 1 extrsion width, previous method may miss some sharp tails.
Now we reduce the width threshold to 0.1*extrusion_width for sharp
tails.
   When checking "nothing below", need to expand the poly a little. Example:
benchy.
2. improve is_support_necessary.
   Only popup warning for sharp tail and large cantilever (longer than 6mm).

Jira: STUDIO-2567
Change-Id: I5e977e7766b35409891d1b41e36278e3f07fa372
(cherry picked from commit c9d8a7c1c3bbd4367c7fc44408c67e5c534167f4)

src/libslic3r/TreeSupport.hpp

@@ -380,6 +380,7 @@ public:
     bool  has_overhangs = false;
     bool  has_sharp_tails = false;
     bool  has_cantilever = false;
+    double max_cantilevel_dist = 0;
     SupportType support_type;
     SupportMaterialStyle support_style;