### 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 XRInterface::PlayAreaMode get_play_area_mode() const override;
	virtual bool set_play_area_mode(XRInterface::PlayAreaMode p_mode) override;
	float get_display_refresh_rate() const;
	void set_display_refresh_rate(float p_refresh_rate);
	Array get_available_display_refresh_rates() const;
	bool is_action_set_active(const String &p_action_set) const;
	void set_action_set_active(const String &p_action_set, bool p_active);
	Array get_action_sets() const;
	double get_render_target_size_multiplier() const;
	void set_render_target_size_multiplier(double multiplier);
	virtual Size2 get_render_target_size() override;
	virtual uint32_t get_view_count() override;
	virtual Transform3D get_camera_transform() override;
	virtual Transform3D get_transform_for_view(uint32_t p_view, const Transform3D &p_cam_transform) override;
	virtual Projection get_projection_for_view(uint32_t p_view, double p_aspect, double p_z_near, double p_z_far) override;
	virtual RID get_color_texture() override;
	virtual RID get_depth_texture() override;
	virtual void process() override;
	virtual void pre_render() override;
	bool pre_draw_viewport(RID p_render_target) override;
	virtual Vector<BlitToScreen> post_draw_viewport(RID p_render_target, const Rect2 &p_screen_rect) override;
	virtual void end_frame() override;
	virtual bool is_passthrough_supported() override;
	virtual bool is_passthrough_enabled() override;
	virtual bool start_passthrough() override;
	virtual void stop_passthrough() override;
	
	virtual Array get_supported_environment_blend_modes() override;
	virtual bool set_environment_blend_mode(XRInterface::EnvironmentBlendMode mode) override;
	void on_state_ready();