Allow custom wxDataViewCtrl renderers to easily use attributes.

Set up the DC passed to wxDataViewCustomRenderer::Render() to use the font and
colour defined by the item attribute by default so that any calls to
RenderText() from it will use them automatically.

Also added public wxDataViewCustomRenderer::GetAttr() to allow retrieving the
attribute explicitly in Render().

The column using custom renderer in the dataview sample now works as expected
in the generic version; the native ones will be corrected in the upcoming
commits.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62590 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk/dataview.cpp

@@ -1268,7 +1268,7 @@ gtk_wx_cell_renderer_render (GtkCellRenderer      *renderer,
             state |= wxDATAVIEW_CELL_INSENSITIVE;
         if (flags & GTK_CELL_RENDERER_FOCUSED)
             state |= wxDATAVIEW_CELL_FOCUSED;
-        cell->Render( renderrect, dc, state );
+        cell->WXCallRender( renderrect, dc, state );
     }
 }