### 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
if (openxr_api == nullptr) {
		return false;
	} else if (!openxr_api->is_initialized()) {
		return false;
	} else {
		return true;
	}
}
bool OpenXRInterface::is_initialized() const {
	return initialized;
};
bool OpenXRInterface::initialize() {
	XRServer *xr_server = XRServer::get_singleton();
	ERR_FAIL_NULL_V(xr_server, false);
	if (openxr_api == nullptr) {
		return false;
	} else if (!openxr_api->is_initialized()) {
		return false;
	} else if (initialized) {
		return true;
	}
	
	_load_action_map();
	if (!openxr_api->initialize_session()) {
		return false;
	}
	
	head.instantiate();
	head->set_tracker_type(XRServer::TRACKER_HEAD);
	head->set_tracker_name("head");