### 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
XrResult result = xrAttachSessionActionSets(session, &attach_info);
	if (XR_FAILED(result)) {
		print_line("OpenXR: failed to attach action sets! [", get_error_string(result), "]");
		return false;
	}
	for (int i = 0; i < p_action_sets.size(); i++) {
		ActionSet *action_set = action_set_owner.get_or_null(p_action_sets[i]);
		ERR_FAIL_NULL_V(action_set, false);
		action_set->is_attached = true;
	}
	/* For debugging:
	print_verbose("Attached set " + action_set->name);
	List<RID> action_rids;
	action_owner.get_owned_list(&action_rids);
	for (int i = 0; i < action_rids.size(); i++) {
		Action * action = action_owner.get_or_null(action_rids[i]);
		if (action && action->action_set_rid == p_action_set) {
			print_verbose(" - Action " + action->name + ": " + OpenXRUtil::get_action_type_name(action->action_type));
			for (int j = 0; j < action->trackers.size(); j++) {
				Tracker * tracker = tracker_owner.get_or_null(action->trackers[j].tracker_rid);
				if (tracker) {
					print_verbose("    - " + tracker->name);
				}
			}
		}
	}
	*/
	return true;
}
void OpenXRAPI::action_set_free(RID p_action_set) {