bw

• threshold: add BMTiled (BiModal Tiled, like Otsu Tiled)
• threshold: BiModal: fix zero approximation
• threshold: uniq functions, remove duplicate code
• replace libpng to fastimage by @plzombie
• filters: add Comix

Build for Debian GNU/Linux by @zvezdochiot .
Build for Windows;flatpak by @plzombie .