use detector extent for shields that is also sensitive to buffer_size - refs #1283

src/agg/process_shield_symbolizer.cpp

@@ -42,7 +42,7 @@ void  agg_renderer<T>::process(shield_symbolizer const& sym,
     shield_symbolizer_helper<face_manager<freetype_engine>,
         label_collision_detector4> helper(
             sym, feature, prj_trans,
-            width_, height_,
+            detector_->extent().width(), detector_->extent().height(),
             scale_factor_,
             t_, font_manager_, *detector_, query_extent_);