Update to FLTK 1.3.8

[Rangi42]

See https://www.fltk.org/articles.php?L1782 and https://groups.google.com/g/fltkgeneral/c/uTOcZwU5Rrg

[Pikrass]

Bumping this. I have high DPI monitors, and Polished Map is very hard to use because of how tiny the UI is. High DPI seems to be supported in FLTK starting from 1.4.0 however, which I guess is harder to switch to.

[Rangi42]

The problem is that there is no 1.4.0 yet. There are weekly 1.4.x snapshots, and their master branch, but still a number of outstanding bugs and incomplete features before it'll be released (and no precise release schedule for it, nor a release candidate).

Also yes, even when 1.4 is out, it doesn't provide automatic good-looking high-DPI support, so I'll have to revise and test Polished Map. I do want to, since almost any computer now is high-DPI, but so far there have been other priorities.

The current scaling for me isn't tiny, but blurry; it may depend on your OS.