Convert .tgs with go libraries (and cgo) (telegram) (#1569)

This commit adds support for go/cgo tgs conversion when building with the -tags `cgo`
The default binaries are still "pure" go and uses the old way of converting.

* Move lottie_convert.py conversion code to its own file

* Add optional libtgsconverter

* Update vendor

* Apply suggestions from code review

* Update bridge/helper/libtgsconverter.go

Co-authored-by: Wim <wim@42.be>

vendor/github.com/Benau/tgsconverter/libtgsconverter/gif.go

@@ -0,0 +1,81 @@
+package libtgsconverter
+
+import "bytes"
+
+import "image"
+import "image/color"
+import "image/gif"
+
+type togif struct {
+	gif gif.GIF
+	images []image.Image
+	prev_frame *image.RGBA
+}
+
+func(to_gif *togif) init(w uint, h uint, options ConverterOptions) {
+	to_gif.gif.Config.Width = int(w)
+	to_gif.gif.Config.Height = int(h)
+}
+
+func(to_gif *togif) SupportsAnimation() bool {
+	return true
+}
+
+func (to_gif *togif) AddFrame(image *image.RGBA, fps uint) error {
+	var fps_int = int(1.0 / float32(fps) * 100.)
+	if to_gif.prev_frame != nil && sameImage(to_gif.prev_frame, image) {
+		to_gif.gif.Delay[len(to_gif.gif.Delay) - 1] += fps_int
+		return nil
+	}
+	to_gif.gif.Image = append(to_gif.gif.Image, nil)
+	to_gif.gif.Delay = append(to_gif.gif.Delay, fps_int)
+	to_gif.gif.Disposal = append(to_gif.gif.Disposal, gif.DisposalBackground)
+	to_gif.images = append(to_gif.images, image)
+	to_gif.prev_frame = image
+	return nil
+}
+
+func (to_gif *togif) Result() []byte {
+	q := medianCutQuantizer{mode, nil, false}
+	p := q.quantizeMultiple(make([]color.Color, 0, 256), to_gif.images)
+	// Add transparent entry finally
+	var trans_idx uint8 = 0
+	if q.reserveTransparent {
+		trans_idx = uint8(len(p))
+	}
+	var id_map = make(map[uint32]uint8)
+	for i, img := range to_gif.images {
+		pi := image.NewPaletted(img.Bounds(), p)
+		for y := 0; y < img.Bounds().Dy(); y++ {
+			for x := 0; x < img.Bounds().Dx(); x++ {
+				c := img.At(x, y)
+				cr, cg, cb, ca := c.RGBA()
+				cid := (cr >> 8) << 16 | cg | (cb >> 8)
+				if q.reserveTransparent && ca == 0 {
+					pi.Pix[pi.PixOffset(x, y)] = trans_idx
+				} else if val, ok := id_map[cid]; ok {
+					pi.Pix[pi.PixOffset(x, y)] = val
+				} else {
+					val := uint8(p.Index(c))
+					pi.Pix[pi.PixOffset(x, y)] = val
+					id_map[cid] = val
+				}
+			}
+		}
+		to_gif.gif.Image[i] = pi
+	}
+	if q.reserveTransparent {
+		p = append(p, color.RGBA{0, 0, 0, 0})
+	}
+	for _, img := range to_gif.gif.Image {
+		img.Palette = p
+	}
+	to_gif.gif.Config.ColorModel = p
+	var data []byte
+	w := bytes.NewBuffer(data)
+	err := gif.EncodeAll(w, &to_gif.gif)
+	if err != nil {
+		return nil
+	}
+	return w.Bytes()
+}