### 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 (p_action.is_null()) {
		return String("None");
	}
	Action *action = action_owner.get_or_null(p_action);
	ERR_FAIL_NULL_V(action, String());
	return action->name;
}
void OpenXRAPI::action_free(RID p_action) {
	Action *action = action_owner.get_or_null(p_action);
	ERR_FAIL_NULL(action);
	if (action->handle != XR_NULL_HANDLE) {
		xrDestroyAction(action->handle);
	}
	action_owner.free(p_action);
}
RID OpenXRAPI::get_interaction_profile_rid(XrPath p_path) {
	List<RID> current;
	interaction_profile_owner.get_owned_list(&current);
	for (int i = 0; i < current.size(); i++) {
		InteractionProfile *ip = interaction_profile_owner.get_or_null(current[i]);
		if (ip && ip->path == p_path) {
			return current[i];
		}
	}
	return RID();
}
XrPath OpenXRAPI::get_interaction_profile_path(RID p_interaction_profile) {