Added some rules to GtkCellAreaBox for rendering the last cell.

- When we reach a cell that is out of the render area, break out
   of the loop (for columns user resized too small)
 - CLAMP the size of the last renderer to fit into the area
   (so that renderers get a chance to ellipsize when rendered
   with a space less than allocation, same reason as above).
 - Hand out remaining space in the render area to the last cell,
   this is for shallow rows in the expand column which may recieve
   more than the allocated width.

tests/testtreeview.c

@@ -351,7 +351,7 @@ set_columns_type (GtkTreeView *tree_view, ColumnsType type)
       gtk_tree_view_set_rules_hint (tree_view, TRUE);
       
       rend = gtk_cell_renderer_text_new ();
-      
+
       col = gtk_tree_view_column_new_with_attributes ("Column 1",
                                                       rend,
                                                       "text", 1,