texturegroup_set_mode

This function allows you to set three things:

Mode: You can set whether Dynamic Texture Groups use Implicit (default) or Explicit loading, by using false for Implicit and true for Explicit.

For information on modes, see: Dynamic Group Modes
Debug: You can enable debugging, which will show you all of your game's Texture Pages along with their status in-game.
Default Sprite: This is the default sprite that appears when the mode is set to Explicit, and the texture you're drawing to draw has not been loaded.

For more information, see: Explicit

texturegroup_set_mode(explicit, [debug=false, default_sprite=-1]);

Argument	Type	Description
explicit	Boolean	The mode used for Dynamic Texture Groups: Implicit (false), or Explicit (true)
debug	Boolean	OPTIONAL Enable/disable Texture Group debugging
default_sprite	Sprite Asset	OPTIONAL Only used in Explicit mode: The sprite used as the "fallback" texture when another texture is not loaded (the whole texture page is drawn)

N/A

texturegroup_set_mode(true, false, spr_fallback);

This will enable Explicit mode, disable debugging and set spr_fallback as the fallback sprite for unloaded textures.