GL contexts and GC blits don't work, dialog placement doesn't work,
and checkbox animations seem to interefere with updating --- but at
least things mostly work.
Relevant to racket/racket#1547
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.
