function svgTxtFarthestPoint(textEl) {
	/** @type {Point} */
	let maxPoint;
	let maxAbsSum = 0;
	for (const span of textEl.getElementsByTagName('tspan')) {
		for (const point of boxPoints(span.getBBox())) {
			const pointAbsSum = Math.abs(point.x) + Math.abs(point.y);
			if (maxAbsSum < pointAbsSum) {
				maxPoint = point;
				maxAbsSum = pointAbsSum;
			}
		}
	}
	return maxPoint;
}
calc farthest point of <tspan>s bbox in {textEl}
origin is in the center
@param {SVGTextElement} textEl