The `gdk_window_ensure_native` call for window freeze and thaw
really is needed, but since it is incompatible with transparent
canvases, don't use freeze and thaw at all for those.
Meanwhile, repair the backing bitmap for both GTK+ 2 and 3
for a transparent canvas when a scale is in effect. And go
back to using X11 bitmaps for backing a canvas on GTK+ 3;
I'm not sure that's the right choice, but probably putting
the bitmap data on the X server instead of client is the
right thing.
Finally, restore GL bitmap support (partly by using X11 bitmaps
to back a canvas).
GL rendering to a canvas with a backing scale is not yet right,
either for GTK+ 2 or 3.
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.
