Returns the [enum FallbackMode] of the autotile. Returns the [enum FallbackMode] of the autotile. The subtile defined as the icon may be used as a fallback when the atlas/autotile's bitmask information is incomplete. It will also be used to represent it in the TileSet editor. Autotiles will automatically find a best match for missing tiles if they're incomplete. Autotiles will use the icon tile for missing tiles if they're incomplete.