cellrenderer GET_SIZE super method

[Kevin Ryde <user42 zip com au>]

I struck a bit of trouble with Gtk2::Ex::Datasheet::DBI not having a
GET_SIZE method in its Gtk2::Ex::Datasheet::DBI::CellRendererText
package.  (RT 36996)

Nosing around the GtkCellRenderer.xs fallback GET_SIZE, I wondered if it
could look up through the classes of the given object instead of using
"caller".  The first superclass which had class->get_size func not equal
to gtk2perl_cell_renderer_get_size would be the one to call, would it?