add getBackgroundImage to ThermionViewerFFI

thermion_dart/lib/src/viewer/src/ffi/src/thermion_viewer_ffi.dart

@@ -199,6 +199,14 @@ class ThermionViewerFFI extends ThermionViewer {
     }
   }
 
+  ///
+  ///
+  ///
+  Future<BackgroundImage> getBackgroundImage() async {
+    _backgroundImage ??= await BackgroundImage.create(this, scene);
+    return _backgroundImage!;
+  }
+
   ///
   ///
   ///