game: use shader for enemy on-hit visual

scenes/enemies/enemy_base.gd

@@ -13,6 +13,7 @@ extends CharacterBody2D
 @onready var nav_agent: NavigationAgent2D = $NavigationAgent2D
 @onready var collision_shape_2d: CollisionShape2D = $CollisionShape2D
 @onready var shape_cast_2d: ShapeCast2D = $ShapeCast2D
+@onready var sprite_2d: Sprite2D = $Sprite2D
 
 var player: Player
 var target: Node2D
@@ -27,6 +28,7 @@ func _ready() -> void:
 	health = max_health
 	shape_cast_2d.shape.radius = collision_shape_2d.shape.radius
 	shape_cast_2d.enabled = false
+	sprite_2d.material = sprite_2d.material.duplicate()
 	_find_player()