Text renders upside down and mirrored

Thank you for the fabulous library! It's very fun to use.

I've noticed that text renders upside down and flipped. I checked against fogleman/gg, which does not have the same issue.

Here’s code to reproduce:

package main

import "github.com/h8gi/canvas"

func main() {
	const w, h = 1024, 768

	c := canvas.NewCanvas(&canvas.CanvasConfig{
		Width:     w,
		Height:    h,
		FrameRate: 60,
	})

	c.Draw(func(ctx *canvas.Context) {
		ctx.SetRGB(1, 1, 1)
		ctx.Clear()
		ctx.SetRGB(0, 0, 0)
		if err := ctx.LoadFontFace("SpaceMono-Regular.ttf", 48); err != nil {
			panic(err)
		}
		ctx.DrawStringAnchored("Hello, world!", w/2, h/2, 0.5, 0.5)

		// Note: this will save the current frame with text rendering correctly.
		// ctx.SavePNG("out.png")
	})
}

Here’s how it renders:

The full source code for the above, which includes the font can be found in this gist. Let me know if you have any questions!

As a note, I haven't been able to locate the problem, but was able to correct text rendering with a transformation matrix on the text portions of the gg.Context. Specifically, something like:

ctx.ScaleAbout(-1, 1, w/2, h/2)
ctx.RotateAbout(math.Pi, w/2, h/2)

This worked for me

	ctx.Push()
	x := 100 // text position
	y := 100 // text position

	ctx.ScaleAbout(-1, 1, x, y)
	ctx.RotateAbout(math.Pi, x, y)

	ctx.SetColor(colornames.White)
	ctx.DrawStringAnchored("Hello wolrd", x, y, 0.5, 0.5)
	ctx.Stroke()
	ctx.Pop()

I think it has something to do with the origin not being at the upper left corner.