### 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
Tracker *tracker = nullptr;
	for (int i = 0; i < trackers.size() && tracker == nullptr; i++) {
		if (trackers[i]->tracker_rid == p_tracker) {
			tracker = trackers[i];
		}
	}
	ERR_FAIL_NULL(tracker);
	tracker->interaction_profile = p_interaction_profile;
	if (p_interaction_profile.is_null()) {
		print_verbose("OpenXR: Interaction profile for " + tracker->tracker_name + " changed to " + INTERACTION_PROFILE_NONE);
		tracker->positional_tracker->set_tracker_profile(INTERACTION_PROFILE_NONE);
	} else {
		String name = openxr_api->interaction_profile_get_name(p_interaction_profile);
		print_verbose("OpenXR: Interaction profile for " + tracker->tracker_name + " changed to " + name);
		tracker->positional_tracker->set_tracker_profile(name);
	}
}
void OpenXRInterface::handle_tracker(Tracker *p_tracker) {