Introduce Viewport.isFullscreenAvailable()

docs/api/viewport.md

 
 A promise that is resolved once the fullscreen mode is no longer active, or rejected on error. The promise will be rejected if the method is called when not in fullscreen mode.
 
+### isFullscreenAvailable
+
+`viewport.isFullscreenAvailable(): boolean`
+
+Checks the availability of the fullscreen mode on the current platform and page.
+
+*Since:* 0.2.1
+
+**Returns**
+
+Returns `true` if the fullscreen mode can be activated.
+
 ## Events
 
 A viewport is an [AREventTarget](ar-event-target.md). You can listen to the following events:

src/core/viewport.ts

     /** Exit fullscreen mode */
     exitFullscreen(): SpeedyPromise<void>;
 
+    /** Is the fullscreen mode available? */
+    isFullscreenAvailable(): boolean;
+
     /** Canvas on which the physical scene will be drawn @internal */
     readonly _backgroundCanvas: HTMLCanvasElement;
 
         });
     }
 
+    /** Is the fullscreen mode available? */
+    isFullscreenAvailable(): boolean
+    {
+        return document.fullscreenEnabled ||
+               !!((document as any).webkitFullscreenEnabled);
+    }
+
     /**
      * True if the viewport is being displayed in fullscreen mode
      */
     }
 
     /**
+     * Is the fullscreen mode available?
+     */
+    isFullscreenAvailable(): boolean
+    {
+        return this._base.fullscreenAvailable;
+    }
+
+    /**
      * Background canvas
      * @internal
      */