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/sizeofint/webpanimation/mux_animi.h

@@ -0,0 +1,43 @@
+// Copyright 2016 Google Inc. All Rights Reserved.
+//
+// Use of this source code is governed by a BSD-style license
+// that can be found in the COPYING file in the root of the source
+// tree. An additional intellectual property rights grant can be found
+// in the file PATENTS. All contributing project authors may
+// be found in the AUTHORS file in the root of the source tree.
+// -----------------------------------------------------------------------------
+//
+// Internal header for animation related functions.
+//
+// Author: Hui Su (huisu@google.com)
+
+#ifndef WEBP_MUX_ANIMI_H_
+#define WEBP_MUX_ANIMI_H_
+
+#include "webp_mux.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+// Picks the optimal rectangle between two pictures, starting with initial
+// values of offsets and dimensions that are passed in. The initial
+// values will be clipped, if necessary, to make sure the rectangle is
+// within the canvas. "use_argb" must be true for both pictures.
+// Parameters:
+//   prev_canvas, curr_canvas - (in) two input pictures to compare.
+//   is_lossless, quality - (in) encoding settings.
+//   x_offset, y_offset, width, height - (in/out) rectangle between the two
+//                                                input pictures.
+// Returns true on success.
+int WebPAnimEncoderRefineRect(
+    const struct WebPPicture* const prev_canvas,
+    const struct WebPPicture* const curr_canvas,
+    int is_lossless, float quality, int* const x_offset, int* const y_offset,
+    int* const width, int* const height);
+
+#ifdef __cplusplus
+}    // extern "C"
+#endif
+
+#endif  // WEBP_MUX_ANIMI_H_