use separate trees for horizontal and curved labels

from e4186926ed38fc1dc232c70098154251a9019669

src/map/map.cpp

@@ -482,6 +482,8 @@ void Map::renderLayers(const std::vector<LayerDescription>& layers, RenderPass p
     } else {
         throw std::runtime_error("unknown render pass");
     }
+
+    glFlush();
 }
 
 template <typename Styles>