Fix compilation warning.

src/shape/convex.cpp

@@ -42,8 +42,8 @@ ConvexBase* ConvexBase::convexHull(const Vec3f* pts, int num_points,
     throw std::logic_error ("Qhull failed");
   }
 
+  typedef std::size_t index_type;
   typedef int size_type;
-  typedef int index_type;
 
   // Map index in pts to index in vertices. -1 means not used
   std::vector<int> pts_to_vertices (num_points, -1);