Add possibility to set item background in generic wxDataViewCtrl.

Added wxDataViewItemAttr::SetBackgroundColour() and code to honour it in the
generic implementation of wxDataViewCtrl.

Closes #12621.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70050 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/dvrenderers.h

@@ -290,6 +290,9 @@ public:
     // platform-specific classes.
     virtual wxDC *GetDC() = 0;
 
+    // To draw background use the background colour in wxDataViewItemAttr
+    virtual void RenderBackground(wxDC* dc, const wxRect& rect);
+
     // Prepare DC to use attributes and call Render().
     void WXCallRender(wxRect rect, wxDC *dc, int state);