### 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
virtual bool initialize() = 0; 
	virtual void uninitialize() = 0; 
	virtual Dictionary get_system_info() = 0; 
	
	virtual PackedStringArray get_suggested_tracker_names() const; 
	virtual PackedStringArray get_suggested_pose_names(const StringName &p_tracker_name) const; 
	virtual TrackingStatus get_tracking_status() const; 
	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); 
	
	virtual bool supports_play_area_mode(XRInterface::PlayAreaMode p_mode); 
	virtual XRInterface::PlayAreaMode get_play_area_mode() const; 
	virtual bool set_play_area_mode(XRInterface::PlayAreaMode p_mode); 
	virtual PackedVector3Array get_play_area() const; 
	
	virtual bool get_anchor_detection_is_enabled() const;
	virtual void set_anchor_detection_is_enabled(bool p_enable);
	virtual int get_camera_feed_id();
	
	virtual Size2 get_render_target_size() = 0; 
	virtual uint32_t get_view_count() = 0; 
	virtual Transform3D get_camera_transform() = 0; 
	virtual Transform3D get_transform_for_view(uint32_t p_view, const Transform3D &p_cam_transform) = 0; 
	virtual Projection get_projection_for_view(uint32_t p_view, double p_aspect, double p_z_near, double p_z_far) = 0; 
	virtual RID get_vrs_texture(); 
	virtual RID get_color_texture(); 
	virtual RID get_depth_texture(); 
	virtual RID get_velocity_texture(); 
	virtual void process() = 0;
	virtual void pre_render(){};
	virtual bool pre_draw_viewport(RID p_render_target) { return true; };