### 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
head->set_tracker_desc("Players head");
	xr_server->add_tracker(head);
	
	Vector<RID> loaded_action_sets;
	for (int i = 0; i < action_sets.size(); i++) {
		loaded_action_sets.append(action_sets[i]->action_set_rid);
	}
	openxr_api->attach_action_sets(loaded_action_sets);
	
	xr_server->set_primary_interface(this);
	initialized = true;
	return initialized;
}
void OpenXRInterface::uninitialize() {
	
	
	
	free_trackers();
	free_interaction_profiles();
	free_action_sets();
	XRServer *xr_server = XRServer::get_singleton();
	if (xr_server) {
		if (head.is_valid()) {
			xr_server->remove_tracker(head);
			head.unref();
		}
	}
	initialized = false;
}
Dictionary OpenXRInterface::get_system_info() {