We've added an experimental option (enabled by default) to keep the
compressed tileset data when we load/save a .aseprite file to avoid
recompressing each time we save (and only compressing the tileset if
tiles are modified).
This is an attempt to make the save operation faster when we use
sprites with several tilemap layers + large tilesets (many tiles, with
big tiles).
Reference: https://github.com/aseprite/Attachment-System/issues/54
