feat make some structs private

src/draw/lv_img_decoder.c

@@ -21,6 +21,17 @@
 /**********************
  *      TYPEDEFS
  **********************/
+typedef struct _lv_img_decoder {
+    lv_img_decoder_info_f_t info_cb;
+    lv_img_decoder_open_f_t open_cb;
+    lv_img_decoder_read_line_f_t read_line_cb;
+    lv_img_decoder_close_f_t close_cb;
+
+#if LV_USE_USER_DATA
+    void * user_data;
+#endif
+} lv_img_decoder_t;
+
 typedef struct {
     lv_fs_file_t f;
     lv_color_t * palette;