game: use shader for enemy on-hit visual

scenes/enemies/enemy_base.tscn

@@ -1,7 +1,12 @@
-[gd_scene load_steps=8 format=3 uid="uid://b7vq8xspnlyeu"]
+[gd_scene load_steps=10 format=3 uid="uid://b7vq8xspnlyeu"]
 
 [ext_resource type="Script" uid="uid://dxn17u7ltuibw" path="res://scenes/enemies/enemy_base.gd" id="1_qty17"]
 [ext_resource type="AnimationLibrary" uid="uid://dos4y853hq1gu" path="res://animation/generic_anims.res" id="2_pkqou"]
+[ext_resource type="Shader" uid="uid://p7evv1wldgsa" path="res://assets/shaders/hit_flash.gdshader" id="2_satqt"]
+
+[sub_resource type="ShaderMaterial" id="ShaderMaterial_qa0nx"]
+shader = ExtResource("2_satqt")
+shader_parameter/flash_amount = 0.0
 
 [sub_resource type="PlaceholderTexture2D" id="PlaceholderTexture2D_pkqou"]
 size = Vector2(32, 32)
@@ -35,6 +40,18 @@ tracks/1/keys = {
 "update": 0,
 "values": [Color(1, 1, 1, 1)]
 }
+tracks/2/type = "value"
+tracks/2/imported = false
+tracks/2/enabled = true
+tracks/2/path = NodePath("Sprite2D:material:shader_parameter/flash_amount")
+tracks/2/interp = 1
+tracks/2/loop_wrap = true
+tracks/2/keys = {
+"times": PackedFloat32Array(0),
+"transitions": PackedFloat32Array(1),
+"update": 0,
+"values": [0.0]
+}
 
 [sub_resource type="AnimationLibrary" id="AnimationLibrary_qa0nx"]
 _data = {
@@ -49,6 +66,7 @@ collision_mask = 3
 script = ExtResource("1_qty17")
 
 [node name="Sprite2D" type="Sprite2D" parent="."]
+material = SubResource("ShaderMaterial_qa0nx")
 texture = SubResource("PlaceholderTexture2D_pkqou")
 
 [node name="CollisionShape2D" type="CollisionShape2D" parent="."]