View / add support for viewport extent constraint

This introduces a breaking change:

The `extent` view option now constrains the whole viewport and not just the
view center.
The option `constrainOnlyCenter` was added to keep the previous behaviour.

Constraining the whole viewport and not only the view center means
that the center and resolution constraints must be applied with a knowledge of
the viewport size.

src/ol/rotationconstraint.js

@@ -5,7 +5,7 @@ import {toRadians} from './math.js';
 
 
 /**
- * @typedef {function((number|undefined), number): (number|undefined)} Type
+ * @typedef {function((number|undefined), number, boolean=): (number|undefined)} Type
  */
 
 
@@ -47,6 +47,7 @@ export function createSnapToN(n) {
     /**
      * @param {number|undefined} rotation Rotation.
      * @param {number} delta Delta.
+     * @param {boolean=} opt_isMoving True if an interaction or animation is in progress.
      * @return {number|undefined} Rotation.
      */
     function(rotation, delta, opt_isMoving) {