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/apng.go

@@ -0,0 +1,51 @@
+package libtgsconverter
+
+import "bytes"
+import "image"
+
+import "github.com/kettek/apng"
+import "github.com/av-elier/go-decimal-to-rational"
+
+type toapng struct {
+	apng apng.APNG
+	prev_frame *image.RGBA
+}
+
+func(to_apng *toapng) init(w uint, h uint, options ConverterOptions) {
+}
+
+func(to_apng *toapng) SupportsAnimation() bool {
+	return true
+}
+
+func (to_apng *toapng) AddFrame(image *image.RGBA, fps uint) error {
+	if to_apng.prev_frame != nil && sameImage(to_apng.prev_frame, image) {
+		var idx = len(to_apng.apng.Frames) - 1
+		var prev_fps = float64(to_apng.apng.Frames[idx].DelayNumerator) / float64(to_apng.apng.Frames[idx].DelayDenominator)
+		prev_fps += 1.0 / float64(fps)
+		rat := dectofrac.NewRatP(prev_fps, 0.001)
+		to_apng.apng.Frames[idx].DelayNumerator = uint16(rat.Num().Int64())
+		to_apng.apng.Frames[idx].DelayDenominator = uint16(rat.Denom().Int64())
+		return nil
+	}
+	f := apng.Frame{}
+	f.Image = image
+	f.DelayNumerator = 1
+	f.DelayDenominator = uint16(fps)
+	f.DisposeOp = apng.DISPOSE_OP_BACKGROUND
+	f.BlendOp = apng.BLEND_OP_SOURCE
+	f.IsDefault = false
+	to_apng.apng.Frames = append(to_apng.apng.Frames, f)
+	to_apng.prev_frame = image
+	return nil
+}
+
+func (to_apng *toapng) Result() []byte {
+	var data []byte
+	w := bytes.NewBuffer(data)
+	err := apng.Encode(w, to_apng.apng)
+	if err != nil {
+		return nil
+	}
+	return w.Bytes()
+}