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

@param {!number} keptWidth
@returns !Rect