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

@param {!number} lostHeight
@returns !Rect