func (f *Fpdf) TransformMirrorVertical(y float64) {
	f.TransformScale(100, -100, f.x, y)
}
TransformMirrorVertical vertically mirrors the following text, drawings and
images. y is the axis of reflection.

The TransformBegin() example demonstrates this method.
package gofpdf
import (
	"fmt"
	"math"
)
type TransformMatrix struct {
	A, B, C, D, E, F float64
}
func (f *Fpdf) TransformBegin() {
	f.transformNest++
	f.out("q")
}
func (f *Fpdf) TransformScaleX(scaleWd, x, y float64) {
	f.TransformScale(scaleWd, 100, x, y)
}
func (f *Fpdf) TransformScaleY(scaleHt, x, y float64) {
	f.TransformScale(100, scaleHt, x, y)
}
func (f *Fpdf) TransformScaleXY(s, x, y float64) {
	f.TransformScale(s, s, x, y)
}
func (f *Fpdf) TransformScale(scaleWd, scaleHt, x, y float64) {
	if scaleWd == 0 || scaleHt == 0 {
		f.err = fmt.Errorf("scale factor cannot be zero")
		return
	}
	y = (f.h - y) * f.k
	x *= f.k
	scaleWd /= 100
	scaleHt /= 100
	f.Transform(TransformMatrix{scaleWd, 0, 0,
		scaleHt, x * (1 - scaleWd), y * (1 - scaleHt)})
}
func (f *Fpdf) TransformMirrorHorizontal(x float64) {
	f.TransformScale(-100, 100, x, f.y)
}