Allow layers that are not managed by the map

When a layer is configured with a map, it will be added on top of other
layers, and not be managed in the map's features collection. The layerState
will have an 'unmanaged' flag for such layers. For vector layers, this flag
is used to not skip any features.

src/ol/renderer/canvas/canvasvectorlayerrenderer.js

@@ -79,7 +79,8 @@ ol.renderer.canvas.VectorLayer.prototype.composeFrame =
   var extent = frameState.extent;
   var focus = frameState.focus;
   var pixelRatio = frameState.pixelRatio;
-  var skippedFeatureUids = frameState.skippedFeatureUids;
+  var skippedFeatureUids = layerState.unmanaged ?
+      {} : frameState.skippedFeatureUids;
   var viewState = frameState.viewState;
   var projection = viewState.projection;
   var rotation = viewState.rotation;