### 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
GDCLASS(OpenXRInterface, XRInterface);
private:
	OpenXRAPI *openxr_api = nullptr;
	bool initialized = false;
	XRInterface::TrackingStatus tracking_state;
	OpenXRFbPassthroughExtensionWrapper *passthrough_wrapper = nullptr;
	
	Ref<XRPositionalTracker> head;
	Transform3D head_transform;
	Vector3 head_linear_velocity;
	Vector3 head_angular_velocity;
	Transform3D transform_for_view[2]; 
	void _load_action_map();
	struct Action { 
		String action_name; 
		OpenXRAction::ActionType action_type; 
		RID action_rid; 
	};
	struct ActionSet { 
		String action_set_name; 
		bool is_active; 
		Vector<Action *> actions; 
		RID action_set_rid; 
	};
	struct Tracker { 
		String tracker_name; 
		Vector<Action *> actions; 
		Ref<XRPositionalTracker> positional_tracker;