application_surface;
Returns: Pointer
This global scope, built-in variable can be used to access the
application surface using any of the surface functions. This
surface is permanently available and is where the bulk of drawing
is done by GameMaker: Studio.
surface_resize(application_surface, display_get_gui_width(), display_get_gui_height());
The above code will resize the application surface to have a 1:1 ratio with the GUI layer.