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/apng.go

@@ -0,0 +1,13 @@
+// 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
+
+type APNG struct {
+	Frames []Frame
+	// LoopCount defines the number of times an animation will be
+	// restarted during display.
+	// A LoopCount of 0 means to loop forever
+	LoopCount uint
+}