### 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
GLOBAL_DEF(PropertyInfo(Variant::STRING, "application/config/windows_native_icon", PROPERTY_HINT_FILE, "*.ico"), String());
	Input *id = Input::get_singleton();
	if (id) {
		agile_input_event_flushing = GLOBAL_DEF("input_devices/buffering/agile_event_flushing", false);
		if (bool(GLOBAL_DEF_BASIC("input_devices/pointing/emulate_touch_from_mouse", false)) &&
				!(editor || project_manager)) {
			if (!DisplayServer::get_singleton()->is_touchscreen_available()) {
				/
				id->set_emulate_touch_from_mouse(true);
			}
		}
		id->set_emulate_mouse_from_touch(bool(GLOBAL_DEF_BASIC("input_devices/pointing/emulate_mouse_from_touch", true)));
	}
	MAIN_PRINT("Main: Load Translations and Remaps");
	translation_server->setup(); 
	if (!locale.is_empty()) {
		translation_server->set_locale(locale);
	}
	translation_server->load_translations();
	ResourceLoader::load_translation_remaps(); 
	ResourceLoader::load_path_remaps();
	MAIN_PRINT("Main: Load TextServer");
	/
	GLOBAL_DEF_RST("internationalization/rendering/text_driver", "");
	String text_driver_options;
	for (int i = 0; i < TextServerManager::get_singleton()->get_interface_count(); i++) {
		const String driver_name = TextServerManager::get_singleton()->get_interface(i)->get_name();
		if (driver_name == "Dummy") {
			/
			continue;