### 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
void tracker_check_profile(RID p_tracker, XrSession p_session = XR_NULL_HANDLE);
	void tracker_free(RID p_tracker);
	RID action_set_create(const String p_name, const String p_localized_name, const int p_priority);
	String action_set_get_name(RID p_action_set);
	bool attach_action_sets(const Vector<RID> &p_action_sets);
	void action_set_free(RID p_action_set);
	RID action_create(RID p_action_set, const String p_name, const String p_localized_name, OpenXRAction::ActionType p_action_type, const Vector<RID> &p_trackers);
	String action_get_name(RID p_action);
	void action_free(RID p_action);
	RID interaction_profile_create(const String p_name);
	String interaction_profile_get_name(RID p_interaction_profile);
	void interaction_profile_clear_bindings(RID p_interaction_profile);
	bool interaction_profile_add_binding(RID p_interaction_profile, RID p_action, const String p_path);
	bool interaction_profile_suggest_bindings(RID p_interaction_profile);
	void interaction_profile_free(RID p_interaction_profile);
	bool sync_action_sets(const Vector<RID> p_active_sets);
	bool get_action_bool(RID p_action, RID p_tracker);
	float get_action_float(RID p_action, RID p_tracker);
	Vector2 get_action_vector2(RID p_action, RID p_tracker);
	XRPose::TrackingConfidence get_action_pose(RID p_action, RID p_tracker, Transform3D &r_transform, Vector3 &r_linear_velocity, Vector3 &r_angular_velocity);
	bool trigger_haptic_pulse(RID p_action, RID p_tracker, float p_frequency, float p_amplitude, XrDuration p_duration_ns);
	void register_composition_layer_provider(OpenXRCompositionLayerProvider *provider);
	void unregister_composition_layer_provider(OpenXRCompositionLayerProvider *provider);
	const XrEnvironmentBlendMode *get_supported_environment_blend_modes(uint32_t &count);
	bool set_environment_blend_mode(XrEnvironmentBlendMode mode);
	OpenXRAPI();
	~OpenXRAPI();
};
#endif