start carrying through the geometry clipping extent in the grid renderer

include/mapnik/grid/grid_renderer.hpp

@@ -119,6 +119,7 @@ private:
     face_manager<freetype_engine> font_manager_;
     label_collision_detector4 detector_;
     boost::scoped_ptr<grid_rasterizer> ras_ptr;
+    box2d<double> query_extent_;
 };
 }
 

src/grid/grid_renderer.cpp

@@ -91,6 +91,7 @@ void grid_renderer<T>::start_layer_processing(layer const& lay, box2d<double> co
     {
         detector_.clear();
     }
+    query_extent_ = query_extent;
 }
 
 template <typename T>