render: extract static M2 batch materializer

src/scenes/streaming/streaming_world_loader.gd

@@ -60,6 +60,9 @@ const M2_PLACEMENT_GROUPER_SCRIPT := preload(
 const M2_BUILD_BATCH_PLANNER_SCRIPT := preload(
 	"res://src/render/m2/m2_build_batch_planner.gd"
 )
+const M2_STATIC_BATCH_MATERIALIZER_SCRIPT := preload(
+	"res://src/render/m2/m2_static_batch_materializer.gd"
+)
 const M2_RUNTIME_MESH_FINALIZER_SCRIPT := preload(
 	"res://src/render/m2/m2_runtime_mesh_finalizer.gd"
 )
@@ -273,6 +276,7 @@ var _m2_unique_placement_registry := (
 var _m2_placement_transform_resolver := M2_PLACEMENT_TRANSFORM_RESOLVER_SCRIPT.new()
 var _m2_placement_grouper := M2_PLACEMENT_GROUPER_SCRIPT.new()
 var _m2_build_batch_planner := M2_BUILD_BATCH_PLANNER_SCRIPT.new()
+var _m2_static_batch_materializer := M2_STATIC_BATCH_MATERIALIZER_SCRIPT.new()
 var _m2_runtime_mesh_finalizer := M2_RUNTIME_MESH_FINALIZER_SCRIPT.new()
 var _m2_raw_model_repository := M2_RAW_MODEL_REPOSITORY_SCRIPT.new()
 var _m2_prototype_cache_state := M2_PROTOTYPE_CACHE_STATE_SCRIPT.new()
@@ -4396,29 +4400,20 @@ func _materialize_m2_group_batch(
 		start: int,
 		count: int,
 		serial: int) -> void:
-	if transforms.is_empty() or count <= 0 or mesh == null:
-		return
-
-	var mm := MultiMesh.new()
-	mm.transform_format = MultiMesh.TRANSFORM_3D
-	mm.mesh = mesh
-	mm.instance_count = count
-	for i in count:
-		mm.set_instance_transform(i, transforms[start + i])
-
-	var mmi := MultiMeshInstance3D.new()
-	mmi.name = "%s_%d" % [rel_path.get_file().get_basename(), serial]
-	mmi.multimesh = mm
-	mmi.cast_shadow = (
-			GeometryInstance3D.SHADOW_CASTING_SETTING_ON
-			if m2_cast_shadows
-			else GeometryInstance3D.SHADOW_CASTING_SETTING_OFF
+	var multimesh_instance := _m2_static_batch_materializer.materialize_batch(
+		m2_root,
+		rel_path,
+		mesh,
+		transforms,
+		start,
+		count,
+		serial,
+		m2_visibility_range,
+		CHUNK_SIZE,
+		m2_cast_shadows
 	)
-	if m2_visibility_range > 0.0:
-		mmi.visibility_range_end = m2_visibility_range
-		mmi.visibility_range_end_margin = CHUNK_SIZE
-	m2_root.add_child(mmi)
-	_set_editor_owner_recursive(mmi)
+	if multimesh_instance != null:
+		_set_editor_owner_recursive(multimesh_instance)
 
 
 func _materialize_m2_animated_batch(