takeBottom(keptHeight) {
        let d = Math.max(keptHeight, 0);
        return new Rect(this.x, this.y + this.h - d, this.w, d);
    }
Returns the result of removing all but the given height from the bottom side of the rectangle.
The cut is clamped so it doesn't go into negative heights.

@param {!number} keptHeight
@returns !Rect