func (gc *GraphicContext) GetStringBounds(s string) (left, top, right, bottom float64) {
	_, h := gc.pdf.GetFontSize()
	d := gc.pdf.GetFontDesc("", "")
	if d.Ascent == 0 {
		// not defined (standard font?), use average of 81%
		top = 0.81 * h
	} else {
		top = -float64(d.Ascent) * h / float64(d.Ascent-d.Descent)
	}
	return 0, top, gc.pdf.GetStringWidth(s), top + h
}
func (gc *GraphicContext) GetStringBounds(s string) (left, top, right, bottom float64) {
	_, h := gc.pdf.GetFontSize()
	d := gc.pdf.GetFontDesc("", "")
	if d.Ascent == 0 {
		// not defined (standard font?), use average of 81%
		top = 0.81 * h
	} else {
		top = -float64(d.Ascent) * h / float64(d.Ascent-d.Descent)
	}
	return 0, top, gc.pdf.GetStringWidth(s), top + h
}
// GetStringBounds returns the approximate pixel bounds of the string s at x, y.
// The left edge of the em square of the first character of s
// and the baseline intersect at 0, 0 in the returned coordinates.
// Therefore the top and left coordinates may well be negative.