Cylindrical panning

I tried out `xpano` for the first time and got a fantastic result!

I got a great cylindrically mapped image with excellent vertical lines. The output matches almost perfectly when looping around the image from right to left… *except* that the colors are not blended across the image edge.

I would like to keep the exact same mapping and crop, but move a different longitude of the cylinder into the center. But when I use the UI to adjust the pan, this makes the image wobble. I can't figure out a way to just horizontally pan while keeping the same mapping and cylindrical crop area.

I tried stripping image metadata and renaming files to try to get them processed in a different order. This *does* affect the longitude that is in the center, but not in a predictable way. Is there any way to control this?

(I can't share the images in this case, but if a repro is important I can try to figure out another set of images to demonstrate with.)

Start by reproducing the cylindrical panorama behavior described in the issue, focusing on UI panning and how the longitude at the image center changes. Done means preserving the existing cylindrical mapping and crop while allowing horizontal panning without wobble and making the selected center longitude predictable.

Written by the indexing model from the issue text.