[core] Localize and simplify state for MapChangeDidFinishLoadingMap event

Only Map needs to track this state, and it can simply be set when beginning to load a style and reset when finished.

include/mbgl/map/map.hpp

@@ -205,6 +205,7 @@ private:
     };
 
     RenderState renderState = RenderState::never;
+    bool loading = false;
 };
 
 } // namespace mbgl