removeShape(shape) {
		// clear the current bounds
		this.bounds.clear();
		// remove the shape from shape list
		remove(this.shapes, shape);
		// add everything left back
		for (let s = 0; s < this.shapes.length; s++) {
			this.addShape(this.shapes[s]);
		}
		// return the length of the shape list
		return this.shapes.length;
	}
remove the specified shape from the body shape list
@param {Polygon|Line|Ellipse} shape - a shape object
@returns {number} the shape array length