Optimize finalized surface artwork

tests/surface_drawing_texture_renderer_validation.gd

@@ -94,6 +94,16 @@ func _run() -> void:
 			if child is MeshInstance3D:
 				artwork_mesh_count += 1
 		assert(artwork_mesh_count == 1)
+		var finalized_instance := canvas.get(
+			"_pixel_instance"
+		) as MeshInstance3D
+		var finalized_material := finalized_instance.mesh.surface_get_material(
+			0
+		) as StandardMaterial3D
+		assert(
+			finalized_material.albedo_texture
+			in persistent_textures
+		)
 		assert(canvas.get_export_image().get_size() == Vector2i(32, 32))
 	world.queue_free()
 	await process_frame