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/go_rlottie/vector_vimageloader.h

@@ -0,0 +1,26 @@
+#ifndef VIMAGELOADER_H
+#define VIMAGELOADER_H
+
+#include <memory>
+
+#include "vector_vbitmap.h"
+
+class VImageLoader
+{
+public:
+    static VImageLoader& instance()
+    {
+         static VImageLoader singleton;
+         return singleton;
+    }
+
+    VBitmap load(const char *fileName);
+    VBitmap load(const char *data, size_t len);
+    ~VImageLoader();
+private:
+    VImageLoader();
+    struct Impl;
+    std::unique_ptr<Impl> mImpl;
+};
+
+#endif // VIMAGELOADER_H