Fix wxDataViewCtrl::GetItemRect() in wxGTK and enable its test

A couple of fixes compared to the previous commit:

- Use the correct gtk_tree_view_get_cell_area() rather than
  gtk_tree_view_get_background_area() which doesn't work correctly
  for the items which are not shown because their parent is collapsed.
- Translate logical coordinates to physical ones using
  gtk_tree_view_convert_bin_window_to_widget_coords().

With these fixes, the unit tests for this function pass and can now be
enabled under wxGTK as well.

See https://github.com/wxWidgets/wxWidgets/pull/990

src/gtk/dataview.cpp

@@ -5304,7 +5304,15 @@ wxDataViewCtrl::GetItemRect(const wxDataViewItem& item,
     wxGtkTreePath path(m_internal->get_path( &iter ));
 
     GdkRectangle item_rect;
-    gtk_tree_view_get_background_area(GTK_TREE_VIEW(m_treeview), path, gcolumn, &item_rect);
+    gtk_tree_view_get_cell_area(GTK_TREE_VIEW(m_treeview), path, gcolumn, &item_rect);
+
+    // GTK returns rectangles with the position and height, but not width, for
+    // some reason, set to 0 if the item is not currently shown, so an explicit
+    // check is needed as this rectangle is not quite the empty rectangle we're
+    // supposed to return in this case.
+    if ( item_rect.height == 0 )
+        return wxRect();
+
     // If column is NULL we compute the combined width of all the columns
     if ( !column )
     {
@@ -5318,6 +5326,33 @@ wxDataViewCtrl::GetItemRect(const wxDataViewItem& item,
         }
         item_rect.width = width;
     }
+
+    // We need to convert logical coordinates to physical ones, i.e. the
+    // rectangle of the topmost item should start at ~0, even if it's a 100th
+    // item shown on top only because the window is scrolled.
+#if GTK_CHECK_VERSION(2, 12, 0)
+    if ( wx_is_at_least_gtk2(12) )
+    {
+        gtk_tree_view_convert_bin_window_to_widget_coords
+        (
+            GTK_TREE_VIEW(m_treeview),
+            item_rect.x, item_rect.y,
+            &item_rect.x, &item_rect.y
+        );
+
+        if ( item_rect.y > GetClientSize().y ||
+                item_rect.y + item_rect.height < 0 )
+        {
+            // If it turns out that the item is not visible at all, indicate it
+            // by returning an empty rectangle for it.
+            return wxRect();
+        }
+    }
+    //else: There doesn't seem to be anything reasonable to do here, so we'll
+    //      just return wrong values with the very old GTK+ versions if the
+    //      window is scrolled.
+#endif // GTK+ 2.12+
+
     return wxRectFromGDKRect(&item_rect);
 }