feat: parse open type features

[nobe4]

This adds support for OpenType Font Features. It does a simple parsing of the input string and uses typesetting/harfbuzz existing method to set the feature.

This makes the font.SetFeature method actually work. TODOs could be removed after that.

E.g.

func main() {
	c := canvas.New(100, 100)
	ctx := canvas.NewContext(c)
	font, err := canvas.LoadFontFile("./examples/font/Fira/FiraSans-Regular.ttf", canvas.FontRegular)
	if err != nil {
		panic(err)
	}

	f, err := os.Create("out.pdf")
	if err != nil {
		panic(err)
	}
	defer f.Close()

	pdf := pdf.New(f, 100, 100, &pdf.Options{})

	features := []string{
		"",
		"subs",
		"tnum",
		"zero",
		"smcp, zero",
	}

	y := 90.0
	for _, feature := range features {
		font.SetFeatures(feature)
		face := font.Face(20.0, canvas.Black)
		txt := canvas.NewTextLine(face, "Aabc ff 3.14 0O "+feature, canvas.Left)
		ctx.DrawText(0, y, txt)
		y -= 10.0
	}

	c.RenderTo(pdf)

	if err := pdf.Close(); err != nil {
		panic(err)
	}
}

Test font used: https://fonts.google.com/specimen/Fira+Sans

[tdewolff]

Awesome, thank you!