roundRect(x, y, width, height, radius) {
		this.moveTo(x + radius, y);
		this.lineTo(x + width - radius, y);
		this.arcTo(x + width, y, x + width, y + radius, radius);

		this.moveTo(x + width, y + radius);
		this.lineTo(x + width, y + height - radius);
		this.arcTo(x + width, y + height, x + width - radius, y + height, radius);

		this.moveTo(x + width - radius, y + height);
		this.lineTo(x + radius, y + height);
		this.arcTo(x, y + height, x, y + height - radius, radius);

		this.moveTo(x, y + height - radius);
		this.lineTo(x, y + radius);
		this.arcTo(x, y, x + radius, y, radius);

		this.isDirty = true;
	}
adds an rounded rectangle to the current path.
@param {number} x - the x-axis coordinate of the rectangle's starting point.
@param {number} y - the y-axis coordinate of the rectangle's starting point.
@param {number} width - the rectangle's width. Positive values are to the right, and negative to the left.
@param {number} height - the rectangle's height. Positive values are down, and negative are up.
@param {number} radius - the arc's radius to draw the borders. Must be positive.