Pan interaction fix

The DragPan and TouchPan interactions just decrements the INTERACTING hint on dragend when *kinetic* is not applied, and no render is requested. See the DragPan's [handleDragEnd func](https://github.com/openlayers/ol3/blob/master/src/ol/interaction/dragpaninteraction.js#L75) for example.

This is a problem, for image layers in particular, as no new image will be requested if kinetic is not applied after panning. You can reproduce the issue with the following steps:

* Load http://openlayers.github.io/ol3/master/examples/wms-single-image-custom-proj.html.
* Zoom in once with the + button.
* Slowly move the map to the north until the current image no longer covers the viewport (white at the bottom of the map container).
* No new image is requested! BUG.

This PR fixes by calling requestRenderFrame when kinetic is not applied and we're no longer interacting.