gltf: fix three bugs which prevented extracted textures from being refreshed.

1. Extracted texture paths in `GLTFDocument::_parse_image_save_image` at the project root started with res:/// which broke cache invalidation
2. md5 hashes were not being written to generator_parameters for new imports, which led Godot to think the file was manually created.
3. `EditorFileSystem::reimport_append` must emit the `resources_reimported` signal in order for the resource cache to be updated.

(cherry picked from commit fea4165ca8e952cef8cb8c636b4399ffdaebbdb5)

editor/editor_file_system.cpp

@@ -2345,7 +2345,13 @@ void EditorFileSystem::reimport_files(const Vector<String> &p_files) {
 
 Error EditorFileSystem::reimport_append(const String &p_file, const HashMap<StringName, Variant> &p_custom_options, const String &p_custom_importer, Variant p_generator_parameters) {
 	ERR_FAIL_COND_V_MSG(!importing, ERR_INVALID_PARAMETER, "Can only append files to import during a current reimport process.");
-	return _reimport_file(p_file, p_custom_options, p_custom_importer, &p_generator_parameters);
+	Error ret = _reimport_file(p_file, p_custom_options, p_custom_importer, &p_generator_parameters);
+
+	// Emit the resource_reimported signal for the single file we just reimported.
+	Vector<String> reloads;
+	reloads.append(p_file);
+	emit_signal(SNAME("resources_reimported"), reloads);
+	return ret;
 }
 
 Error EditorFileSystem::_resource_import(const String &p_path) {

modules/gltf/gltf_document.cpp

@@ -3126,7 +3126,7 @@ void GLTFDocument::_parse_image_save_image(Ref<GLTFState> p_state, const String
 			bool must_import = true;
 			Vector<uint8_t> img_data = p_image->get_data();
 			Dictionary generator_parameters;
-			String file_path = p_state->get_base_path() + "/" + p_state->filename.get_basename() + "_" + p_image->get_name() + ".png";
+			String file_path = p_state->get_base_path().path_join(p_state->filename.get_basename() + "_" + p_image->get_name()) + ".png";
 			if (FileAccess::exists(file_path + ".import")) {
 				Ref<ConfigFile> config;
 				config.instantiate();
@@ -3137,6 +3137,8 @@ void GLTFDocument::_parse_image_save_image(Ref<GLTFState> p_state, const String
 				if (!generator_parameters.has("md5")) {
 					must_import = false; // Didn't come from a gltf document; don't overwrite.
 				}
+			}
+			if (must_import) {
 				String existing_md5 = generator_parameters["md5"];
 				unsigned char md5_hash[16];
 				CryptoCore::md5(img_data.ptr(), img_data.size(), md5_hash);