Make it possible to share the memory between several gifio.OnDiskGif objects

[deshipu]

Since the gifio.OnDiskGif always uses RGB565 format for its bitmap, instead a indexed image as it should, it uses a lot of memory (four times more than it would do for a 16-color image normally). To work around that for large gifs, it would be nice to display several smaller gif images instead, but share the bitmap object, or at least its memory buffer, between the OnDiskGif instances, so that we could update several smaller images, wait for the next frame, update them again, wait for the next frame, etc.

One simple way of doing it would be to allow to pass the bitmap object to the OnDiskGif when it's created, so instead of creating a new bitmap, it would reuse the one being passed. There are probably other ways of doing it.