### 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 (OpenXRCompositionLayerProvider *provider : composition_layer_providers) {
		memdelete(provider);
	}
	composition_layer_providers.clear();
	if (supported_extensions != nullptr) {
		memfree(supported_extensions);
		supported_extensions = nullptr;
	}
	if (layer_properties != nullptr) {
		memfree(layer_properties);
		layer_properties = nullptr;
	}
#ifdef ANDROID_ENABLED
	if (openxr_loader_library_handle) {
		OS::get_singleton()->close_dynamic_library(openxr_loader_library_handle);
		openxr_loader_library_handle = nullptr;
	}
#endif
	singleton = nullptr;
}
Transform3D OpenXRAPI::transform_from_pose(const XrPosef &p_pose) {
	Quaternion q(p_pose.orientation.x, p_pose.orientation.y, p_pose.orientation.z, p_pose.orientation.w);
	Basis basis(q);
	Vector3 origin(p_pose.position.x, p_pose.position.y, p_pose.position.z);
	return Transform3D(basis, origin);
}
template <typename T>
XRPose::TrackingConfidence _transform_from_location(const T &p_location, Transform3D &r_transform) {