### 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
RID tracker_rid; 
		RID interaction_profile; 
	};
	Vector<ActionSet *> action_sets;
	Vector<RID> interaction_profiles;
	Vector<Tracker *> trackers;
	ActionSet *create_action_set(const String &p_action_set_name, const String &p_localized_name, const int p_priority);
	void free_action_sets();
	Action *create_action(ActionSet *p_action_set, const String &p_action_name, const String &p_localized_name, OpenXRAction::ActionType p_action_type, const Vector<Tracker *> p_trackers);
	Action *find_action(const String &p_action_name);
	void free_actions(ActionSet *p_action_set);
	Tracker *find_tracker(const String &p_tracker_name, bool p_create = false);
	void handle_tracker(Tracker *p_tracker);
	void free_trackers();
	void free_interaction_profiles();
	void _set_default_pos(Transform3D &p_transform, double p_world_scale, uint64_t p_eye);
protected:
	static void _bind_methods();
public:
	virtual StringName get_name() const override;
	virtual uint32_t get_capabilities() const override;
	virtual PackedStringArray get_suggested_tracker_names() const override;
	virtual TrackingStatus get_tracking_status() const override;
	bool initialize_on_startup() const;
	virtual bool is_initialized() const override;
	virtual bool initialize() override;
	virtual void uninitialize() override;
	virtual Dictionary get_system_info() override;
	virtual void trigger_haptic_pulse(const String &p_action_name, const StringName &p_tracker_name, double p_frequency, double p_amplitude, double p_duration_sec, double p_delay_sec = 0) override;
	virtual bool supports_play_area_mode(XRInterface::PlayAreaMode p_mode) override;