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/webp.go

@@ -0,0 +1,39 @@
+package libtgsconverter
+
+import "bytes"
+import "image"
+
+import "github.com/sizeofint/webpanimation"
+
+type towebp struct {
+	timestamp int
+	webpanim *webpanimation.WebpAnimation
+	config webpanimation.WebPConfig
+}
+
+func(to_webp *towebp) init(w uint, h uint, options ConverterOptions) {
+	to_webp.timestamp = 0
+	to_webp.webpanim = webpanimation.NewWebpAnimation(int(w), int(h), 0)
+	to_webp.config = webpanimation.NewWebpConfig()
+	to_webp.config.SetQuality(options.GetWebpQuality())
+}
+
+func(to_webp *towebp) SupportsAnimation() bool {
+	return true
+}
+
+func (to_webp *towebp) AddFrame(image *image.RGBA, fps uint) error {
+	err := to_webp.webpanim.AddFrame(image, to_webp.timestamp, to_webp.config)
+	to_webp.timestamp += int((1.0 / float32(fps)) * 1000.)
+	return err
+}
+
+func (to_webp *towebp) Result() []byte {
+	var buf bytes.Buffer
+	err := to_webp.webpanim.Encode(&buf)
+	if err != nil {
+		return nil
+	}
+	to_webp.webpanim.ReleaseMemory()
+	return buf.Bytes()
+}