### 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
Vector<ActionTracker> trackers; 
		XrAction handle; 
	};
	RID_Owner<Action, true> action_owner;
	RID get_action_rid(XrAction p_action);
	struct InteractionProfile { 
		String name; 
		XrPath path; 
		Vector<XrActionSuggestedBinding> bindings; 
	};
	RID_Owner<InteractionProfile, true> interaction_profile_owner;
	RID get_interaction_profile_rid(XrPath p_path);
	XrPath get_interaction_profile_path(RID p_interaction_profile);
	
	bool poll_events();
	bool on_state_idle();
	bool on_state_ready();
	bool on_state_synchronized();
	bool on_state_visible();
	bool on_state_focused();
	bool on_state_stopping();
	bool on_state_loss_pending();
	bool on_state_exiting();
	
	void copy_string_to_char_buffer(const String p_string, char *p_buffer, int p_buffer_len);
public:
	XrInstance get_instance() const { return instance; };
	XrSystemId get_system_id() const { return system_id; };
	XrSession get_session() const { return session; };
	String get_runtime_name() const { return runtime_name; };