### 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
#endif
	Engine::get_singleton()->set_frame_delay(frame_delay);
	message_queue = memnew(MessageQueue);
	Thread::release_main_thread(); 
	set_current_thread_safe_for_nodes(false);
	if (p_second_phase) {
		return setup2();
	}
	OS::get_singleton()->benchmark_end_measure("core");
	return OK;
error:
	text_driver = "";
	display_driver = "";
	audio_driver = "";
	tablet_driver = "";
	Engine::get_singleton()->set_write_movie_path(String());
	project_path = "";
	args.clear();
	main_args.clear();
	if (show_help) {
		print_help(execpath);
	}
	EngineDebugger::deinitialize();
	if (performance) {
		memdelete(performance);
	}
	if (input_map) {
		memdelete(input_map);