Pockets for the Crazyhouse GIFs

[notruck]

The new "Share as a GIF" option is almost instantaneous, and it's such a useful feature. Thank you for that!

The current implementation seems to work well for most variants. It would be nice if the Crazyhouse GIFs can display the pockets too, Also, because of the extra information the viewers will be having to process, perhaps the replay speed for zh images could be made slightly slower than the others?

Thank you for your time and consideration!

[niklasf]

Performance: Thanks, hearing that makes me happy! I spent quite a bit of time on it.

Timing: There's some WIP on taking the actual move times into account. The open question is how to scale or cap it, so that slower games do not feel too slow.

Crazyhouse: I don't plan to add support myself, but if someone wants to write some Rust to add support for pockets, here are the steps:

1. Add pocket pieces to the SVG sprite (https://github.com/niklasf/lila-gif/blob/master/theme/sprite.svg).
2. Use Gimp to render with indexed color palette (currently 63 colors) and square size 90, updating the GIF sprite (https://github.com/niklasf/lila-gif/blob/master/theme/sprite.gif).
3. Add a getter to select the pocket sprite to Theme (similar to https://github.com/niklasf/lila-gif/blob/1d27cc1fcd2b5b91a22850b3e69082ed1e94d0f5/src/theme.rs#L104-L108).
4. Actually render pockets (https://github.com/niklasf/lila-gif/blob/master/src/render.rs). params.fen already has the required Crazyhouse pocket data: https://docs.rs/shakmaty/0.16.2/shakmaty/fen/struct.Fen.html#structfield.pockets