[core, darwin, android] Add onDidEnterIdle to MapObserver.

didEnterIdle fires whenever render completes and no repaint is scheduled.

include/mbgl/map/map_observer.hpp

@@ -39,6 +39,7 @@ public:
     virtual void onDidFinishRenderingMap(RenderMode) {}
     virtual void onDidFinishLoadingStyle() {}
     virtual void onSourceChanged(style::Source&) {}
+    virtual void onDidEnterIdle() {}
 };
 
 } // namespace mbgl