Add options to further reduce memory use

[Undoundoundo]

When processing large (dimensions, frame count) GIF files, as each frame is added to the .frames array - even with the rawData/deinterlacedData purged - a lot of memory ends up used by the plain data and even the structure of a frame itself.

It would be nice to have an option to not store frame data at all, leaving only the binary, the main GIF object and the requested frame.
This does mean that if a frame is requested again, it will have to be re-processed, but processing is fairly fast and with workers, implementations could buffer a sequence of frames, as well as store key frames at an interval to avoid having to re-build from frame 0.