### 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
Ref<XRPositionalTracker> positional_tracker;
	positional_tracker.instantiate();
	
	if (p_tracker_name == "/user/hand/left") {
		positional_tracker->set_tracker_type(XRServer::TRACKER_CONTROLLER);
		positional_tracker->set_tracker_name("left_hand");
		positional_tracker->set_tracker_desc("Left hand controller");
		positional_tracker->set_tracker_hand(XRPositionalTracker::TRACKER_HAND_LEFT);
	} else if (p_tracker_name == "/user/hand/right") {
		positional_tracker->set_tracker_type(XRServer::TRACKER_CONTROLLER);
		positional_tracker->set_tracker_name("right_hand");
		positional_tracker->set_tracker_desc("Right hand controller");
		positional_tracker->set_tracker_hand(XRPositionalTracker::TRACKER_HAND_RIGHT);
	} else {
		positional_tracker->set_tracker_type(XRServer::TRACKER_CONTROLLER);
		positional_tracker->set_tracker_name(p_tracker_name);
		positional_tracker->set_tracker_desc(p_tracker_name);
	}
	positional_tracker->set_tracker_profile(INTERACTION_PROFILE_NONE);
	xr_server->add_tracker(positional_tracker);
	
	tracker = memnew(Tracker);
	tracker->tracker_name = p_tracker_name;
	tracker->tracker_rid = tracker_rid;
	tracker->positional_tracker = positional_tracker;
	tracker->interaction_profile = RID();
	trackers.push_back(tracker);
	return tracker;
}
void OpenXRInterface::tracker_profile_changed(RID p_tracker, RID p_interaction_profile) {