Adjust a workaround for versions before 3.22 when setting the font for
a control.
GTK+ version 3.22 starts paying attention to whether a font size for a
control is absolute (as opposed to being in points), so the workaround
that was put in place for earlier versions breaks.
In addition, some part of the drawing stack seems to round point sizes
to an integeral size after DPI conversion. Take that rounding into
account when setting the font size in `normal-control-font`.
Closes#1522
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.
