### 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
} else {
			action->action_name = p_action_name;
		}
	} else {
		action->action_name = p_action_name;
	}
	action->action_type = p_action_type;
	action->action_rid = openxr_api->action_create(p_action_set->action_set_rid, p_action_name, p_localized_name, p_action_type, tracker_rids);
	p_action_set->actions.push_back(action);
	
	for (int i = 0; i < p_trackers.size(); i++) {
		if (p_trackers[i]->actions.find(action) == -1) {
			p_trackers[i]->actions.push_back(action);
		}
	}
	return action;
}
OpenXRInterface::Action *OpenXRInterface::find_action(const String &p_action_name) {
	
	for (int i = 0; i < action_sets.size(); i++) {
		for (int j = 0; j < action_sets[i]->actions.size(); j++) {
			if (action_sets[i]->actions[j]->action_name == p_action_name) {
				return action_sets[i]->actions[j];
			}
		}
	}
	
	return nullptr;
}
void OpenXRInterface::free_actions(ActionSet *p_action_set) {