[core] Split MapMode::Still into Static and Tile

`Tile` makes sure the symbols in the resulting tile are tileable while
symbols in `Still` match rendering in `Continuous` mode.

platform/node/src/node_map.hpp

@@ -74,6 +74,7 @@ public:
     std::unique_ptr<mbgl::AsyncRequest> request(const mbgl::Resource&, mbgl::FileSource::Callback);
 
     const float pixelRatio;
+    mbgl::MapMode mode;
     NodeThreadPool threadpool;
     NodeMapObserver mapObserver;
     std::unique_ptr<mbgl::HeadlessFrontend> frontend;