[kitty/gif] provide auto detection of background for perception of transparency

[acxz]

For the kitty terminal, in particular, when viewing a gif a background color needs to be provided to ensure consequent frames are not overlayed ontop of one another.

For gifs that are transparent, this breaks the transparency. One way to recover the perception of transparency is to automatically determine the background of the current terminal and use that color as the background of the gif that is displayed.

Note this issue only appears when displaying gifs in the kitty terminal.

This could be implemented as the following option: --bg auto.

Alternatives: timg solves this issue with the same flag -b auto, which they keep as default.

See the conversation following this comment for more rational/info/disc.

[hpjansson]

Progress:

• c3f932b adds the necessary seqs to the backend.
• df1fdf0 adds response parsing.

I still need to write a parser class that can separate an input stream into seqs and regular text input, and a terminal class to do asynchronous I/O.