Refactor constraints static methods to module functions

src/ol/centerconstraint.js

@@ -0,0 +1,37 @@
+/**
+ * @module ol/centerconstraint
+ */
+import {clamp} from './math.js';
+
+
+/**
+ * @param {ol.Extent} extent Extent.
+ * @return {ol.CenterConstraintType} The constraint.
+ */
+export function createExtent(extent) {
+  return (
+    /**
+     * @param {ol.Coordinate|undefined} center Center.
+     * @return {ol.Coordinate|undefined} Center.
+     */
+    function(center) {
+      if (center) {
+        return [
+          clamp(center[0], extent[0], extent[2]),
+          clamp(center[1], extent[1], extent[3])
+        ];
+      } else {
+        return undefined;
+      }
+    }
+  );
+}
+
+
+/**
+ * @param {ol.Coordinate|undefined} center Center.
+ * @return {ol.Coordinate|undefined} Center.
+ */
+export function none(center) {
+  return center;
+}