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/kettek/apng/frame.go

@@ -0,0 +1,47 @@
+// Copyright 2018 kts of kettek / Ketchetwahmeegwun Tecumseh Southall. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package apng
+
+import (
+	"image"
+)
+
+// dispose_op values, as per the APNG spec.
+const (
+	DISPOSE_OP_NONE       = 0
+	DISPOSE_OP_BACKGROUND = 1
+	DISPOSE_OP_PREVIOUS   = 2
+)
+
+// blend_op values, as per the APNG spec.
+const (
+	BLEND_OP_SOURCE = 0
+	BLEND_OP_OVER   = 1
+)
+
+type Frame struct {
+	Image              image.Image
+	width, height    int
+	XOffset, YOffset int
+	DelayNumerator   uint16
+	DelayDenominator uint16
+	DisposeOp        byte
+	BlendOp          byte
+	// IsDefault indicates if the Frame is a default image that
+	// should not be used in the animation. IsDefault can only
+	// be true on the first frame.
+	IsDefault bool
+}
+
+// GetDelay returns the number of seconds in the frame.
+func (f *Frame) GetDelay() float64 {
+	d := uint16(0)
+	if f.DelayDenominator == 0 {
+		d = 100
+	} else {
+		d = f.DelayDenominator
+	}
+	return float64(f.DelayNumerator) / float64(d)
+}