Better support for unicode/fonts

[ZwerOxotnik]

It seems like, it doesn't treat properly some characters. I'm using fonts from https://www.kreativekorp.com/ucsur/

I did added symbols to make_font_textures.sh for https://www.kreativekorp.com/software/fonts/sitelenselikiwen/ and tested with it:

# F1900−F19FF SITELEN PONA
generate f1900 f19ff
# F1C80−F1C9F SITELEN PONA RADICALS
generate f1c80 f1c9f
# FF900−FFBED SITELEN PONA COMPOUNDS (probably wrong)
generate ff900 ffbed
# F1C40−F1C7F TITI PULA
generate f1c40 f1c7f

It uses *_0000.png on signs instead.