Update wxSlider background when its parent background changes in wxMSW.

The native control doesn't redraw itself, so force it to do it from the
overridden DoMSWControlColor() which is called every time the background
colour might have changed.

See #12271.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76985 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/slider.h

@@ -118,9 +118,15 @@ protected:
     virtual void DoMoveWindow(int x, int y, int width, int height);
     virtual wxSize DoGetBestSize() const;
 
+    WXHBRUSH DoMSWControlColor(WXHDC pDC, wxColour colBg, WXHWND hWnd) wxOVERRIDE;
+
+
     // the labels windows, if any
     wxSubwindows  *m_labels;
 
+    // Last background brush we returned from DoMSWControlColor(), see there.
+    WXHBRUSH m_hBrushBg;
+
     int           m_rangeMin;
     int           m_rangeMax;
     int           m_pageSize;