### Godot version

4.1

### System information

Windows 10 - Godot 4.1.1 - Compatibility

### Issue description

When trying to use the Varjo XR3 headset (in OpenXR mode) with Godot 4 (with OpenXR and passthrough activated), the passthrough does not work, i.e. the scene is rendered on a black background instead of the camera stream.

It seems to me that are two problems in the openxr module, more precisely in the openx_api.cpp file (fixing them solves  the issue) : 

1. The XR_COMPOSITION_LAYER_BLEND_TEXTURE_SOURCE_ALPHA_BIT  is only set when the number of layers is > 1, which is fine for the Meta Quest headset but does not work for the Varjo since it only requires one layer. 
2. The environment blend mode XR_ENVIRONMENT_BLEND_MODE_ALPHA_BLEND  is only really set when the XRFbPassthrough extension is present, which is not the case with the Varjo runtime. 

I am not sure how to fix this without causing issues for other headsets.

1. Would it be an issue to always activate the XR_COMPOSITION_LAYER_BLEND_TEXTURE_SOURCE_ALPHA_BIT ? 
2. Could blend modes be activated even if they are not supported by an extension ? 





### Steps to reproduce

- Create a Godot 4 project with OpenXR activated in Compatibility mode
- Add the code to activate OpenXr and to enable passthrough : https://docs.godotengine.org/en/stable/tutorials/xr/openxr_passthrough.html#passthrough-extension
- Start the Varjo Base software and activate OpenXR
- Run the Godot project

### Minimal reproduction project

N/A
initialize_modules(MODULE_INITIALIZATION_LEVEL_EDITOR);
	GDExtensionManager::get_singleton()->initialize_extensions(GDExtension::INITIALIZATION_LEVEL_EDITOR);
	ClassDB::set_current_api(ClassDB::API_CORE);
#endif
	MAIN_PRINT("Main: Load Modules");
	register_platform_apis();
	GLOBAL_DEF_BASIC(PropertyInfo(Variant::STRING, "display/mouse_cursor/custom_image", PROPERTY_HINT_FILE, "*.png,*.webp"), String());
	GLOBAL_DEF_BASIC("display/mouse_cursor/custom_image_hotspot", Vector2());
	GLOBAL_DEF_BASIC("display/mouse_cursor/tooltip_position_offset", Point2(10, 10));
	if (String(GLOBAL_GET("display/mouse_cursor/custom_image")) != String()) {
		Ref<Texture2D> cursor = ResourceLoader::load(
				GLOBAL_GET("display/mouse_cursor/custom_image"));
		if (cursor.is_valid()) {
			Vector2 hotspot = GLOBAL_GET("display/mouse_cursor/custom_image_hotspot");
			Input::get_singleton()->set_custom_mouse_cursor(cursor, Input::CURSOR_ARROW, hotspot);
		}
	}
	camera_server = CameraServer::create();
	MAIN_PRINT("Main: Load Physics");
	initialize_physics();
	initialize_navigation_server();
	register_server_singletons();
	/
	ScriptServer::init_languages();
	theme_db->initialize_theme();
	audio_server->load_default_bus_layout();
#if defined(MODULE_MONO_ENABLED) && defined(TOOLS_ENABLED)
	/
	/
	/