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/imagewriter.go

@@ -0,0 +1,40 @@
+package libtgsconverter
+
+import "image"
+
+type imageWriter interface {
+	init(w uint, h uint, options ConverterOptions)
+	SupportsAnimation() bool
+	AddFrame(image *image.RGBA, fps uint) error
+	Result() []byte
+}
+
+func sameImage(a *image.RGBA, b *image.RGBA) bool {
+	if len(a.Pix) != len(b.Pix) {
+		return false
+	}
+	for i, v := range a.Pix {
+		if v != b.Pix[i] {
+			return false
+		}
+	}
+	return true
+}
+
+func newImageWriter(extension string, w uint, h uint, options ConverterOptions) imageWriter {
+	var writer imageWriter
+	switch extension {
+	case "apng":
+		writer = &toapng{}
+	case "gif":
+		writer = &togif{}
+	case "png":
+		writer = &topng{}
+	case "webp":
+		writer = &towebp{}
+	default:
+		return nil
+	}
+	writer.init(w, h, options)
+	return writer
+}