### 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
#ifndef XR_INTERFACE_H
#define XR_INTERFACE_H
#include "core/math/projection.h"
#include "core/os/thread_safe.h"
#include "servers/xr_server.h"
struct BlitToScreen;
/**
	The XR interface is a template class on top of which we build interface to different AR, VR and tracking SDKs.
	The idea is that we subclass this class, implement the logic, and then instantiate a singleton of each interface
	when Godot starts. These instances do not initialize themselves but register themselves with the AR/VR server.
	If the user wants to enable AR/VR the choose the interface they want to use and initialize it.
	Note that we may make this into a fully instantiable class for GDExtension support.
*/
class XRInterface : public RefCounted {
	GDCLASS(XRInterface, RefCounted);
public:
	enum Capabilities { 
		XR_NONE = 0, 
		XR_MONO = 1, 
		XR_STEREO = 2, 
		XR_QUAD = 4, 
		XR_VR = 8, 
		XR_AR = 16, 
		XR_EXTERNAL = 32 
	};
	enum TrackingStatus {