Converts PDF rectangle to the viewport coordinates.
@param rect {Array} xMin, yMin, xMax and yMax coordinates.
@returns {Array} Contains corresponding coordinates of the rectangle
in the viewport coordinate space.
@see {@link convertToViewportPoint}
function PageViewport_convertToViewportRectangle(rect) {
      var tl = Util.applyTransform([rect[0], rect[1]], this.transform);
      var br = Util.applyTransform([rect[2], rect[3]], this.transform);
      return [tl[0], tl[1], br[0], br[1]];
    }