### 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 = tracker_owner.get_or_null(p_trackers[i]);
		if (tracker != nullptr && tracker->toplevel_path != XR_NULL_PATH) {
			ActionTracker action_tracker = {
				p_trackers[i], 
				XR_NULL_HANDLE, 
				false 
			};
			action.trackers.push_back(action_tracker);
			toplevel_paths.push_back(tracker->toplevel_path);
		}
	}
	XrActionCreateInfo action_info = {
		XR_TYPE_ACTION_CREATE_INFO, 
		nullptr, 
		"", 
		action.action_type, 
		uint32_t(toplevel_paths.size()), 
		toplevel_paths.ptr(), 
		"" 
	};
	copy_string_to_char_buffer(p_name, action_info.actionName, XR_MAX_ACTION_NAME_SIZE);
	copy_string_to_char_buffer(p_localized_name, action_info.localizedActionName, XR_MAX_LOCALIZED_ACTION_NAME_SIZE);
	XrResult result = xrCreateAction(action_set->handle, &action_info, &action.handle);
	if (XR_FAILED(result)) {
		print_line("OpenXR: failed to create action ", p_name, "! [", get_error_string(result), "]");
		return RID();
	}
	return action_owner.make_rid(action);
}
String OpenXRAPI::action_get_name(RID p_action) {