### 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
String get_runtime_version() const { return runtime_version; };
	
	Transform3D transform_from_pose(const XrPosef &p_pose);
	
	XRPose::TrackingConfidence transform_from_location(const XrSpaceLocation &p_location, Transform3D &r_transform);
	XRPose::TrackingConfidence transform_from_location(const XrHandJointLocationEXT &p_location, Transform3D &r_transform);
	void parse_velocities(const XrSpaceVelocity &p_velocity, Vector3 &r_linear_velocity, Vector3 &r_angular_velocity);
	bool xr_result(XrResult result, const char *format, Array args = Array()) const;
	bool is_top_level_path_supported(const String &p_toplevel_path);
	bool is_interaction_profile_supported(const String &p_ip_path);
	bool interaction_profile_supports_io_path(const String &p_ip_path, const String &p_io_path);
	static bool openxr_is_enabled(bool p_check_run_in_editor = true);
	_FORCE_INLINE_ static OpenXRAPI *get_singleton() { return singleton; }
	XrResult try_get_instance_proc_addr(const char *p_name, PFN_xrVoidFunction *p_addr);
	XrResult get_instance_proc_addr(const char *p_name, PFN_xrVoidFunction *p_addr);
	String get_error_string(XrResult result);
	String get_swapchain_format_name(int64_t p_swapchain_format) const;
	void set_xr_interface(OpenXRInterface *p_xr_interface);
	static void register_extension_wrapper(OpenXRExtensionWrapper *p_extension_wrapper);
	static void unregister_extension_wrapper(OpenXRExtensionWrapper *p_extension_wrapper);
	static void register_extension_metadata();
	static void cleanup_extension_wrappers();
	void set_form_factor(XrFormFactor p_form_factor);
	XrFormFactor get_form_factor() const { return form_factor; }
	void set_view_configuration(XrViewConfigurationType p_view_configuration);
	XrViewConfigurationType get_view_configuration() const { return view_configuration; }
	void set_reference_space(XrReferenceSpaceType p_reference_space);
	XrReferenceSpaceType get_reference_space() const { return reference_space; }
	void set_submit_depth_buffer(bool p_submit_depth_buffer);
	bool get_submit_depth_buffer() const { return submit_depth_buffer; }