### 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
Basis basis;
	Vector3 origin;
	XRPose::TrackingConfidence confidence = XRPose::XR_TRACKING_CONFIDENCE_NONE;
	const XrPosef &pose = p_location.pose;
	
	if (p_location.locationFlags & XR_SPACE_LOCATION_ORIENTATION_VALID_BIT) {
		Quaternion q(pose.orientation.x, pose.orientation.y, pose.orientation.z, pose.orientation.w);
		r_transform.basis = Basis(q);
		if (p_location.locationFlags & XR_SPACE_LOCATION_ORIENTATION_TRACKED_BIT) {
			
			confidence = XRPose::XR_TRACKING_CONFIDENCE_HIGH;
		} else {
			
			confidence = XRPose::XR_TRACKING_CONFIDENCE_LOW;
		}
	} else {
		r_transform.basis = Basis();
	}
	
	if (p_location.locationFlags & XR_SPACE_LOCATION_POSITION_VALID_BIT) {
		r_transform.origin = Vector3(pose.position.x, pose.position.y, pose.position.z);
		if (!(p_location.locationFlags & XR_SPACE_LOCATION_ORIENTATION_TRACKED_BIT)) {
			
			confidence = XRPose::XR_TRACKING_CONFIDENCE_LOW;
		} else if (confidence == XRPose::XR_TRACKING_CONFIDENCE_NONE) {