Names a state's animation, given whatever varies it - a facing direction for a grid
character, an empty string for one that does not need it. Looked up through
AnimatedSprite.has, so a state with nothing registered for the current variant is
silently skipped rather than throwing - the same degrade-to-nothing GridMover already
relies on for a sprite with no walk cycle at all.
Optionalvariantthe variant to start in; defaults to the empty string
Example