Interaction hints

This pull request adds a generic system for interactions to pass hints to the renderer. Currently it is only used by `ol.DragPanInteraction` and `ol.renderer.dom.TileLayerRenderer` to prevent the removal of DOM elements while panning, à la Leaflet. Further hints can be added as needed.

In the future it is conceivable that other objects other than interactions may set these hints (e.g. animations), and that more hints may be added (e.g. rotating, zooming). These can be added on an as-needed basis.