Possible off-by-one error in calculating the AImage `displayIndex` array.

[proxpero]

I have a two-frame gif and only the first frame shows. With a three-frame gif only two frames animate back and forth. One frame is being dropped. I think it has something to do with the displayIndex array on AImage, but I can't understand the logic in the calculateFrameDelay function well enough to figure out exactly what is causing the issue.