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.

test/spec/ol/interaction/interaction.test.js

@@ -132,6 +132,7 @@ describe('ol.interaction.Interaction', function() {
       const view = new View({
         center: [0, 0],
         extent: [-2.5, -2.5, 2.5, 2.5],
+        constrainOnlyCenter: true,
         resolution: 1,
         resolutions: [4, 2, 1, 0.5, 0.25]
       });

test/spec/ol/view.test.js

@@ -45,7 +45,8 @@ describe('ol.View', function() {
       describe('with extent option', function() {
         it('gives a correct center constraint function', function() {
           const options = {
-            extent: [0, 0, 1, 1]
+            extent: [0, 0, 1, 1],
+            constrainOnlyCenter: true
           };
           const fn = createCenterConstraint(options);
           expect(fn([0, 0])).to.eql([0, 0]);