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 `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.
For GTK+ 2, we used `gdk_window_ensure_native` to make sure
that freeze and thaw work. For GTK+ 3, `gdk_window_ensure_native`
seems not to be needed and prevents a canvas from drawing over
its background.
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.
Support GUI scaling in much the same way as on Windows, where
the OS setting ("org.gnome.desktop.interface.scaling-factor"
times "...text-scaling-factor") determines the scale that is
used for both graphics and GUI sizing.
As I understand it, a complete solution requires porting to
Gtk3. With Gtk2, the graphical part of a widget doesn't scale.
Text and image labels should scale correctly, though.
