FIX: thumbnail is not clear in dark mode

JIRA: STUDIO-5087

Change-Id: Ie86493ed71b5554095927f061509a1f551758b61
Signed-off-by: wenjie.guo <wenjie.guo@bambulab.com>

src/slic3r/GUI/SelectMachine.hpp

@@ -514,6 +514,9 @@ public:
     void PaintBackground(wxDC &dc);
     void OnEraseBackground(wxEraseEvent &event);
     void set_thumbnail(wxImage img);
+    void paint(wxPaintEvent& evt);
+private:
+    ScalableBitmap m_background_bitmap;
     
 };