message: cgltf_writer: fix inconsistent buffer size.
  `extension_flags` was uninitialized, which caused intermittent junk in
  the output.
diff: @@ -623,12 +623,15 @@ static void cgltf_write_accessor(cgltf_write_context* context, const cgltf_acces
  cgltf_result cgltf_write_file(const cgltf_options* options, const char* path, const cgltf_data* data)
  {
  - size_t size = cgltf_write(options, NULL, 0, data);
  - char* buffer = (char*) malloc(size);
  - size = cgltf_write(options, buffer, size, data);
  + size_t expected = cgltf_write(options, NULL, 0, data);
  + char* buffer = (char*) malloc(expected);
  + size_t actual = cgltf_write(options, buffer, expected, data);
  + if (expected != actual) {
  + fprintf(stderr, "Error: expected %zu bytes but wrote %zu bytes.\n", expected, actual);
  + }
  FILE* file = fopen(path, "wt");
  // Note that cgltf_write() includes a null terminator, which we omit from the file content.
  - fwrite(buffer, size - 1, 1, file);
  + fwrite(buffer, actual - 1, 1, file);
  fclose(file);
  free(buffer);
  return cgltf_result_success;
  @@ -646,6 +649,7 @@ cgltf_size cgltf_write(const cgltf_options* options, char* buffer, cgltf_size si
  ctx.depth = 1;
  ctx.indent = " ";
  ctx.needs_comma = 0;
  + ctx.extension_flags = 0;
  cgltf_write_context* context = &ctx;
  