demos depend on graphics environment

[lhf]

The demos included in libschrift depend on the X11 graphics environment. I've written a longer, exploratory demo that does not: instead, it generates a PGM file, which can be viewed virtually everywhere. See https://github.com/lhf/libschrift-show.
Feel free to use it any way you see fit.

Thank you for libschrift!

[tomolt]

Thanks! I'll take a closer look when I find some time.