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

didEnterIdle fires whenever render completes and no repaint is scheduled.

src/mbgl/map/map.cpp

@@ -209,6 +209,8 @@ void Map::Impl::onDidFinishRenderingFrame(RenderMode renderMode, bool needsRepai
 
         if (needsRepaint || transform.inTransition()) {
             onUpdate();
+        } else if (rendererFullyLoaded) {
+            observer.onDidEnterIdle();
         }
     } else if (stillImageRequest && rendererFullyLoaded) {
         auto request = std::move(stillImageRequest);