### 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
if (OS::get_singleton()->get_environment("USER") == "root" && !OS::get_singleton()->has_environment("GODOT_SILENCE_ROOT_WARNING")) {
		WARN_PRINT("Started the engine as `root`/superuser. This is a security risk, and subsystems like audio may not work correctly.\nSet the environment variable `GODOT_SILENCE_ROOT_WARNING` to 1 to silence this warning.");
	}
#endif
	OS::get_singleton()->initialize_joypads();
	
	AudioDriverManager::initialize(audio_driver_idx);
	print_line(" "); 
	
	audio_server = memnew(AudioServer);
	audio_server->init();
	
	xr_server = memnew(XRServer);
	register_core_singletons();
	MAIN_PRINT("Main: Setup Logo");
#if !defined(TOOLS_ENABLED) && (defined(WEB_ENABLED) || defined(ANDROID_ENABLED))
	bool show_logo = false;
#else
	bool show_logo = true;
#endif
	if (init_windowed) {
		
	} else if (init_maximized) {
		DisplayServer::get_singleton()->window_set_mode(DisplayServer::WINDOW_MODE_MAXIMIZED);
	} else if (init_fullscreen) {
		DisplayServer::get_singleton()->window_set_mode(DisplayServer::WINDOW_MODE_FULLSCREEN);
	}
	if (init_always_on_top) {
		DisplayServer::get_singleton()->window_set_flag(DisplayServer::WINDOW_FLAG_ALWAYS_ON_TOP, true);