Work around wrong offset in plugin windows

src/wine-host/editor.h

@@ -311,6 +311,11 @@ class Editor {
      */
     void redetect_host_window() noexcept;
 
+    /**
+     * Get offset of parent window to fix mouse coordinates.
+     */
+    std::array<int16_t, 2> get_parent_window_offset();
+
     /**
      * Reparent `child` to `new_parent`. This includes the flush.
      */