skipLeft(lostWidth) {
        let d = Math.min(lostWidth, this.w);
        return new Rect(this.x + d, this.y, this.w - d, this.h);
    }
Returns the result of removing the given width from the left side of the rectangle.
The cut is clamped so it doesn't go into negative heights.

@param {!number} lostWidth
@returns !Rect