Hide viewport elements when releasing it

src/core/hud.ts

      */
     _release(): void
     {
+        this.visible = false;
+
         if(this._isOwnContainer) {
             this._isOwnContainer = false;
             this._container.remove();

src/core/viewport.ts

      */
     release(): void
     {
+        this._backgroundCanvas.hidden = true;
+        this._foregroundCanvas.hidden = true;
+
         this._backgroundCanvas.style.cssText = '';
         this._foregroundCanvas.style.cssText = this._originalCSSTextOfForegroundCanvas;
     }