CharacterSpriteBehavior: Fallback to idle when walk is not available

When the SpriteFrames animation doesn't have an animation named "walk", use its
"idle" animation for walking. This makes the modding easier, since a single
animation is now needed for start testing a SpriteFrames in a character with any
of the walk behaviors. We also use "idle" at double speed for running.

And also, when a character is made of multiple parts, some parts can stay in the
"idle" animation. Meaning there are less parts to draw.

Author: manuq

scenes/game_logic/sprite_behaviors/character_sprite_behavior.gd

@@ -56,10 +56,10 @@ func _process_animations(_delta: float) -> void:
 				sprite.play(&"run")
 			else:
 				sprite.speed_scale = 2.0
-				sprite.play(&"walk")
+				sprite.play(&"walk" if sprite.sprite_frames.has_animation(&"walk") else &"idle")
 		else:
 			sprite.speed_scale = 1.0
-			sprite.play(&"walk")
+			sprite.play(&"walk" if sprite.sprite_frames.has_animation(&"walk") else &"idle")
 
 
 func _notification(what: int) -> void: