Register max_width to png format

diff --git a/src/imageio/format/png.c b/src/imageio/format/png.c
index 719af59..af067b5 100644
--- a/src/imageio/format/png.c
+++ b/src/imageio/format/png.c
@@ -406,6 +406,7 @@ void init(dt_imageio_module_format_t *self)
 #ifdef USE_LUA
   luaA_struct(darktable.lua_state,dt_imageio_png_t);
   dt_lua_register_module_member(darktable.lua_state,self,dt_imageio_png_t,bpp,int);
+  dt_lua_register_module_member(darktable.lua_state,self,dt_imageio_png_t,max_width,int);
 #endif
 }
 void cleanup(dt_imageio_module_format_t *self) {}