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/PrintObject.cpp

@@ -2629,6 +2629,7 @@ SupportNecessaryType PrintObject::is_support_necessary()
     }
 #else
     TreeSupport tree_support(*this, m_slicing_params);
+    tree_support.support_type = SupportType::stTreeAuto; // need to set support type to fully utilize the power of feature detection
     tree_support.detect_overhangs();
     this->clear_support_layers();
     if (tree_support.has_sharp_tails)