show image on server without X display installed
1. on a server without X-display installed
2. adjust the terminal size,
more small, more clear, more slow
3. python3 catimg.py xxxxx.png
4. cat _xxxxx.png.txt
5. the result generated in a very 7-point fontsize terminal
just map the bgr color-space to ansi-256 color-space handled pixel-by-pixel, so its slow................