Allow post-import plugins to modify _subresources

The old code fetched some data before the `EditorScenePostImportPlugin._pre_process` callback.
While the callback could modify existing keys, this prevented users from adding new data on a fresh import.

By fetching the keys after pre_process, this means users can consistently modify import options for nodes, meshes, materials and animations in a post-import plugin.

Author: lyuma

doc/classes/EditorScenePostImportPlugin.xml

@@ -71,6 +71,7 @@
 			<param index="0" name="scene" type="Node" />
 			<description>
 				Pre Process the scene. This function is called right after the scene format loader loaded the scene and no changes have been made.
+				Pre process may be used to adjust internal import options in the [code]"nodes"[/code], [code]"meshes"[/code], [code]"animations"[/code] or [code]"materials"[/code] keys inside [code]get_option_value("_subresources")[/code].
 			</description>
 		</method>
 		<method name="add_import_option">

editor/import/3d/resource_importer_scene.cpp

@@ -2934,38 +2934,22 @@ Error ResourceImporterScene::import(ResourceUID::ID p_source_id, const String &p
 
 	Dictionary subresources = p_options["_subresources"];
 
-	Dictionary node_data;
-	if (subresources.has("nodes")) {
-		node_data = subresources["nodes"];
-	}
-
-	Dictionary material_data;
-	if (subresources.has("materials")) {
-		material_data = subresources["materials"];
-	}
-
-	Dictionary animation_data;
-	if (subresources.has("animations")) {
-		animation_data = subresources["animations"];
-	}
-
-	Dictionary mesh_data;
-	if (subresources.has("meshes")) {
-		mesh_data = subresources["meshes"];
-	}
-
 	Error err = OK;
 
 	// Check whether any of the meshes or animations have nonexistent save paths
 	// and if they do, fail the import immediately.
-	err = _check_resource_save_paths(mesh_data);
-	if (err != OK) {
-		return err;
+	if (subresources.has("meshes")) {
+		err = _check_resource_save_paths(subresources["meshes"]);
+		if (err != OK) {
+			return err;
+		}
 	}
 
-	err = _check_resource_save_paths(animation_data);
-	if (err != OK) {
-		return err;
+	if (subresources.has("animations")) {
+		err = _check_resource_save_paths(subresources["animations"]);
+		if (err != OK) {
+			return err;
+		}
 	}
 
 	List<String> missing_deps; // for now, not much will be done with this
@@ -3005,6 +2989,27 @@ Error ResourceImporterScene::import(ResourceUID::ID p_source_id, const String &p
 		post_importer_plugins.write[i]->pre_process(scene, p_options);
 	}
 
+	// data in _subresources may be modified by pre_process(), so wait until now to check.
+	Dictionary node_data;
+	if (subresources.has("nodes")) {
+		node_data = subresources["nodes"];
+	}
+
+	Dictionary material_data;
+	if (subresources.has("materials")) {
+		material_data = subresources["materials"];
+	}
+
+	Dictionary animation_data;
+	if (subresources.has("animations")) {
+		animation_data = subresources["animations"];
+	}
+
+	Dictionary mesh_data;
+	if (subresources.has("meshes")) {
+		mesh_data = subresources["meshes"];
+	}
+
 	float fps = 30;
 	if (p_options.has(SNAME("animation/fps"))) {
 		fps = (float)p_options[SNAME("animation/fps")];