#71 Cache previously viewed image

[Ed Avis]

Jpegview uses a separate thread to process the following image in the background, so you can quickly move to it. It seems to follow the last direction chosen. But a common usage, for me at least, is to flip back and forth between a pair of images. Which is better - this one or that one? Currently this is rather slow because jpegview recalculates each time. It doesn't have the previously viewed image cached.

Jpegview should cache the last viewed image so you can quickly flip back to it.

[Ed Avis]