fix: correct computed sizes on slice window to properly display margins

The display size of the Output image was being computed using the available element size, which included margins and was leading to the bottom couple pixels being clipped. Correcting that to properly account for the actual image size.

src/gui/image_slice_window.hpp

@@ -137,7 +137,8 @@ private:
   
   // --------------------------------------------------- : Events
   DECLARE_EVENT_TABLE();
-  
+  
+  wxSize getBestSliceSize() const;
   wxSize DoGetBestSize() const override;
   
   void onLeftDown(wxMouseEvent&);