### 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
for (OpenXRExtensionWrapper *extension_wrapper : registered_extension_wrappers) {
		memdelete(extension_wrapper);
	}
	registered_extension_wrappers.clear();
}
Size2 OpenXRAPI::get_recommended_target_size() {
	ERR_FAIL_NULL_V(view_configuration_views, Size2());
	Size2 target_size;
	target_size.width = view_configuration_views[0].recommendedImageRectWidth * render_target_size_multiplier;
	target_size.height = view_configuration_views[0].recommendedImageRectHeight * render_target_size_multiplier;
	return target_size;
}
XRPose::TrackingConfidence OpenXRAPI::get_head_center(Transform3D &r_transform, Vector3 &r_linear_velocity, Vector3 &r_angular_velocity) {
	XrResult result;
	if (!running) {
		return XRPose::XR_TRACKING_CONFIDENCE_NONE;
	}
	
	if (frame_state.predictedDisplayTime == 0) {
		return XRPose::XR_TRACKING_CONFIDENCE_NONE;
	}
	
	XrTime display_time = get_next_frame_time();
	XrSpaceVelocity velocity = {
		XR_TYPE_SPACE_VELOCITY, 
		nullptr, 
		0, 
		{ 0.0, 0.0, 0.0 }, 
		{ 0.0, 0.0, 0.0 } 
	};