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_stb_stb_image.cpp

@@ -0,0 +1,59 @@
+/*
+ * configure stb_image about
+ * the image we will support
+ */
+#define STB_IMAGE_IMPLEMENTATION
+
+#define STBI_ONLY_JPEG
+#define STBI_ONLY_PNG
+#define STBI_NO_HDR
+#define STBI_NO_LINEAR
+#define STBI_NO_GIF
+#define STBI_NO_PIC
+
+#include "vector_stb_stb_image.h"
+
+#if defined _WIN32 || defined __CYGWIN__
+  #ifdef RLOTTIE_BUILD
+    #define RLOTTIE_API __declspec(dllexport)
+  #else
+    #define RLOTTIE_API __declspec(dllimport)
+  #endif
+#else
+  #ifdef RLOTTIE_BUILD
+      #define RLOTTIE_API __attribute__ ((visibility ("default")))
+  #else
+      #define RLOTTIE_API
+  #endif
+#endif
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/*
+ * exported function wrapper from the library
+ */
+
+RLOTTIE_API unsigned char *lottie_image_load(char const *filename, int *x,
+                                            int *y, int *comp, int req_comp)
+{
+    return stbi_load(filename, x, y, comp, req_comp);
+}
+
+RLOTTIE_API unsigned char *lottie_image_load_from_data(const char *imageData,
+                                                      int len, int *x, int *y,
+                                                      int *comp, int req_comp)
+{
+    unsigned char *data = (unsigned char *)imageData;
+    return stbi_load_from_memory(data, len, x, y, comp, req_comp);
+}
+
+RLOTTIE_API void lottie_image_free(unsigned char *data)
+{
+    stbi_image_free(data);
+}
+
+#ifdef __cplusplus
+}
+#endif