View / add getValidCenter method to improve interactions

The DragPan, KeyboardPan and DragZoom interactions now make sure to
animate to a valid center/resolution target to avoid
a chained "resolve" animation which looks weird.

The `View.fit` method was also fixed to use this.

src/ol/interaction/Interaction.js

@@ -114,7 +114,7 @@ export function pan(view, delta, opt_duration) {
     view.animate({
       duration: opt_duration !== undefined ? opt_duration : 250,
       easing: linear,
-      center: center
+      center: view.getValidCenter(center)
     });
   }
 }