Correct bug with items without attributes in wxGTK wxDVC.

After the change to the sample in r62390 it turned out that wxGTK version
didn't handle items without attributes in a column where other items did have
attributes neither -- they inherited the last used attribute.

Fix this by remembering whether we are using any non-default attributes or not
and resetting them if we do.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62392 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/dataview.h

@@ -42,10 +42,17 @@ public:
     void GtkInitHandlers();
     void GtkUpdateAlignment();
 
+    bool GtkIsUsingDefaultAttrs() const { return m_usingDefaultAttrs; }
+    void GtkSetUsingDefaultAttrs(bool def) { m_usingDefaultAttrs = def; }
+
 protected:
     GtkCellRenderer   *m_renderer;
     int                m_alignment;
 
+    // true if we hadn't changed any visual attributes or restored them since
+    // doing this
+    bool m_usingDefaultAttrs;
+
 protected:
     DECLARE_DYNAMIC_CLASS_NO_COPY(wxDataViewRenderer)
 };