Mosh: SIXEL graphics support

[rongmu]

SIXEL is an image format that allows high-quality image display in terminal.
For detailed information, check libsixel repo and Wikipedia:

• https://github.com/saitoha/libsixel
• https://en.wikipedia.org/wiki/Sixel

As shown in the comparison below, it seems that sixel graphics are supported by SSH but not mosh.
The terminal emulator tested with is wsl-terminal v0.8.13, which is based on mintty. On Linux, mlterm also has sixel support out-of-the-box.

SSH

mosh

[ebetica]

I can confirm this on xterm compiled with sixel support as well. Would be amazing to have this!