### 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
ERR_FAIL_COND_V(instance == XR_NULL_HANDLE, RID());
	Tracker new_tracker;
	new_tracker.name = p_name;
	new_tracker.toplevel_path = XR_NULL_PATH;
	new_tracker.active_profile_rid = RID();
	XrResult result = xrStringToPath(instance, p_name.utf8().get_data(), &new_tracker.toplevel_path);
	if (XR_FAILED(result)) {
		print_line("OpenXR: failed to get path for ", p_name, "! [", get_error_string(result), "]");
		return RID();
	}
	return tracker_owner.make_rid(new_tracker);
}
String OpenXRAPI::tracker_get_name(RID p_tracker) {
	if (p_tracker.is_null()) {
		return String("None");
	}
	Tracker *tracker = tracker_owner.get_or_null(p_tracker);
	ERR_FAIL_NULL_V(tracker, String());
	return tracker->name;
}
void OpenXRAPI::tracker_check_profile(RID p_tracker, XrSession p_session) {