Add sixel backend

[shuni64]

Closes #121
This adds decent support for sixel graphics. I only tested it on xterm, but it should work on other terminal emulators too. There seems to be a 16 color limit, so dithering is used to make them look somewhat okay. The kitty backend also needed a fix because the thread workaround caused it to read one more byte from stdin. It wasn't fatal, but would've caused weird behaviour later on, so I replaced it with posix calls.

[o2sh]

@CephalonRho Don't you think, it would be better to let the user choose which image_backend to use instead of having a fallback to the first one that is supported by the terminal?

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Still haven't tested it, but I'm sure it works great!!

[o2sh]

mlterm (version 3.8.6) >>

xterm(330) >>

[o2sh]

Still doesn't work on my xterm, which version do you use?

Beside, I don't think that the two step process of first setting the image backend and then being able to use it is the best way to do it.

Let's make it simpler:

onefetch --sixel ./my-picture.png
onefetch --kitty ./my-picture.png

and no fallback (get_best_backend), if the specified backend isn't supported, just show the regular output (ascii_logo + git_infos), or just the git_info (right column) and no ascii_logo.

[shuni64]

I'm using xterm(350). It should be launched with "xterm -ti vt340" to enable sixel support, but if it's not enabled then it should've panicked unless --image-backend was specified.

Not sure if requiring the user to know which backend their terminal emulator supports is a great idea. We can detect the backend that will definitely work best on the used emulator automatically, so I don't see a reason to not do that. Right now there shouldn't be a reason to use the --image-backend argument other than the detection failing (for example on yaft), so it wouldn't be a two step process most of the time.

[o2sh]

It should be launched with "xterm -ti vt340" to enable sixel support

That dit it, thx!

it should've panicked unless --image-backend was specified.

No, it didn't panicked, just a funky rendering (see screenshot above)

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Ok, for the automatic image_backend detection.

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By the way, I don't think the image-backend flag works, when I'm on a Kitty terminal and set the image backend to sixel, it stills render the image using kitty... I'm pretty sure the overriden value is lost in-between two execution.

I feel like this is the kind of setting that should be put into a static file and read at compile time/Run time, not a command line flag.

If we stick to the automatic detection, you can rollback to the first version, unless I'm missing something

[shuni64]

I forgot to include the part that actually detects sixel support when removing the thread. That's hopefully fixed now.

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The image-backend flag works fine for me.

It prints lots of garbage to the screen, but that's expected when using the sixel backend on a terminal emulator that doesn't support sixel.
The flag does not persist across executions, and unless I missed something there's no config file to implement that with. It would probably be weird for scripts to use anyway.

[o2sh]

LGTM 👍

Thx a lot,

Looking forward to your next contribution @CephalonRho