code: function (x, y, width, height, dpr) {
          var viewport;
          if (typeof x === 'object') {
              viewport = x;
          }
          else {
              viewport = {
                  x: x, y: y,
                  width: width, height: height,
                  devicePixelRatio: dpr || 1
              };
          }
          this._frameBuffer.viewport = viewport;
      }
docs: TODO is dpr needed?