make background (judgement-line) animatable

Shaders/Cutout2D.gdshader

@@ -0,0 +1,15 @@
+shader_type canvas_item;
+
+uniform sampler2D mask;
+
+void fragment( )
+{
+	vec4 mask_color = texture(mask, UV).rgba;
+	vec4 sprite_color = texture(TEXTURE, UV).rgba;
+	
+	if (mask_color.a == 0.0)
+	{
+		sprite_color.a = 0.0;
+	}
+	COLOR = sprite_color;
+}