Add filter transparency-by-color (e.g. greenscreen)

[Zulko]

Motivation:

Some GIFs, videos and images can have a green (or black or white) background. Users may want to make that background transparent so they can overlay the asset over another one.

Blocked by: #33

Specs

• In the Asset Filters (not yet implemented as of writing), users can define a new filter where they define the color to make transparent.
• There may be a need for another parameter defining the sharpness of the filter to apply.

Technical notes

• There doesn't seem to be any library providing the effect, but this may be doable with something like Jimp
• Unclear how much this would slow down previews if done in real time. An alternative (at least for gifs and videos with few frames) could be to cache the green-screen frames so they only get computed once.