Add a GIF optimization (=size-reduction) feature

[Zulko]

Motivation

Gix enables you to generate GIFs, but these are generally unoptimized and heavy. Would be nice if we offered the possibility to optimize the GIFs, so they are more easily sharable.

Specs

• Ideally the optimization step would come after the GIF rendering. Like, the user is presented with a GIF that is said to weight 6Mo, and there is a new button that says "Optimize GIF size" that the user can click on.
• If the implementation relies on a heavy framework, this should only be downloaded when the user uses the feature for the first time.

Technical details

There might not be any existing framework for that 😬 but possible paths include:

• Lobbying for this project to create a webasm browser build of gifsicle (looks like they did most of the heavy lifting already).
• Looking into ffmpeg-for-browser (not sure it supports GIFs already. But desktop ffmpeg creates lighter gifs).