FIX: unsupported cantilever was not warned

1. fix unsupported cantilever was not warned. We need to set support
  type to stTreeAuto in is_support_necessary() to fully utilize the
  power of feature detection.
2. fix tree support not generated on non-first plate.

Change-Id: I5b8aa827c9d62df0119db4e8e9212f33ce75ddaa
(cherry picked from commit 60f21b6fcb45fe56f7fc867a7fcad433282bc055)

src/libslic3r/TreeSupport.hpp

@@ -369,6 +369,7 @@ public:
     bool  has_overhangs = false;
     bool  has_sharp_tails = false;
     bool  has_cantilever = false;
+    SupportType support_type;
 
     std::unique_ptr<FillLightning::Generator> generator;
     std::unordered_map<double, size_t> printZ_to_lightninglayer;