Restore the use of freeze & thaw on transparent canvases, but
ensure that the implementation underlying the GDK window won't
change while a freeze is in effect.
The main advantage of GTK+ 3 is better support for HiDPI
displays. If GTK+ 3 libraries are not available or if the
`PLT_GTK2` environment variable is defined, GTK+ 2 is used
as before.