fix compiler warning (also pass by value since it's faster for PODs)

poly2tri/sweep/sweep_context.cc

@@ -103,7 +103,7 @@ void SweepContext::InitEdges(std::vector<Point*> polyline)
   }
 }
 
-Point* SweepContext::GetPoint(const int& index)
+Point* SweepContext::GetPoint(size_t index)
 {
   return points_[index];
 }

poly2tri/sweep/sweep_context.h

@@ -77,7 +77,7 @@ void MapTriangleToNodes(Triangle& t);
 
 void AddToMap(Triangle* triangle);
 
-Point* GetPoint(const int& index);
+Point* GetPoint(size_t index);
 
 Point* GetPoints();