Cleanup on viewer destroy and register on Viewer()

Other changes:

- ESLint comment about undeclared variable Map removed, since Map
  is now registered as a global in .eslintrc.*
  // eslint-disable-next-line no-undef
- Mark as private the property _viewers of OpenSeadragon

src/openseadragon.js

@@ -749,9 +749,7 @@
 
 /* eslint-disable no-redeclare */
 function OpenSeadragon( options ){
-    var viewer = new OpenSeadragon.Viewer( options );
-    OpenSeadragon._viewers.set(viewer.element, viewer);
-    return viewer;
+    return new OpenSeadragon.Viewer( options );
 }
 
 (function( $ ){
@@ -1409,10 +1407,10 @@ function OpenSeadragon( options ){
          * Keep track of which {@link Viewer}s have been created.
          * - Key: {@link Element} to which a Viewer is attached.
          * - Value: {@link Viewer} of the element defined by the key.
+         * @private
          * @static
          * @type {Object}
          */
-        // eslint-disable-next-line no-undef
         _viewers: new Map(),
 
        /**

src/viewer.js

@@ -481,6 +481,8 @@ $.Viewer = function( options ) {
         this.drawer.setImageSmoothingEnabled(this.imageSmoothingEnabled);
     }
 
+    // Register the viewer
+    $._viewers.set(this.element, this);
 };
 
 $.extend( $.Viewer.prototype, $.EventSource.prototype, $.ControlDock.prototype, /** @lends OpenSeadragon.Viewer.prototype */{
@@ -824,6 +826,9 @@ $.extend( $.Viewer.prototype, $.EventSource.prototype, $.ControlDock.prototype,
         this.canvas = null;
         this.container = null;
 
+        // Unregister the viewer
+        $._viewers.delete(this.element);
+
         // clear our reference to the main element - they will need to pass it in again, creating a new viewer
         this.element = null;
     },