View / add a method to compute a valid zoom level

The `getValidZoomLevel` apply the current resolution constraint to return
a value that is guaranteed valid.

This is used for interactions & controls which need a target value to work:
the +/- buttons, the zoom clider, the dragbox zoom and the mouse wheel zoom.

src/ol/control/Zoom.js

@@ -114,20 +114,20 @@ class Zoom extends Control {
       // upon it
       return;
     }
-    const currentResolution = view.getResolution();
-    if (currentResolution) {
-      const newResolution = view.constrainResolution(currentResolution, delta);
+    const currentZoom = view.getZoom();
+    if (currentZoom !== undefined) {
+      const newZoom = view.getValidZoomLevel(currentZoom + delta);
       if (this.duration_ > 0) {
         if (view.getAnimating()) {
           view.cancelAnimations();
         }
         view.animate({
-          resolution: newResolution,
+          zoom: newZoom,
           duration: this.duration_,
           easing: easeOut
         });
       } else {
-        view.setResolution(newResolution);
+        view.setZoom(newZoom);
       }
     }
   }

src/ol/control/ZoomSlider.js

@@ -218,9 +218,10 @@ class ZoomSlider extends Control {
       event.offsetY - this.thumbSize_[1] / 2);
 
     const resolution = this.getResolutionForPosition_(relativePosition);
+    const zoom = view.getValidZoomLevel(view.getZoomForResolution(resolution));
 
     view.animate({
-      resolution: view.constrainResolution(resolution),
+      zoom: zoom,
       duration: this.duration_,
       easing: easeOut
     });
@@ -281,8 +282,11 @@ class ZoomSlider extends Control {
       const view = this.getMap().getView();
       view.endInteraction();
 
+      const zoom = view.getValidZoomLevel(
+        view.getZoomForResolution(this.currentResolution_));
+
       view.animate({
-        resolution: view.constrainResolution(this.currentResolution_),
+        zoom: zoom,
         duration: this.duration_,
         easing: easeOut
       });