make textures_dump_atlases work again and expose it as a utility function

include/twn_util.h

@@ -68,6 +68,10 @@ TWN_API int64_t file_to_bytes(const char *path, unsigned char **buf_out);
 TWN_API char *file_to_str(const char *path);
 
 
+/* saves all texture atlases as BMP files in the write directory */
+TWN_API void textures_dump_atlases(void);
+
+
 /* returns true if str ends with suffix */
 TWN_API TWN_API bool strends(const char *str, const char *suffix);