game: add shader to indicate low hp

scenes/player.gd

@@ -51,6 +51,7 @@ func take_damage(value: float) -> void:
 	dm.damage_taken = value
 	dm.player_damage = false
 	add_child(dm)
+	GlobalConst.sig_on_player_hp_change.emit(current_hp, max_hp)
 	main_ui.player_ui.update_hp()
 	if player_stats.current_health <= 0:
 		die()