Option to keep offset when switching file

[piko-piko]

I've added an option --keep-offset to preserve the forced offset (set by --offset) when switching to a next file in a slideshow

I was trying to adapt feh for use as a lightweight manga reader and images getting centered to the center of the page were preventing that use case.

[derf]

How does this differ from --keep-zoom-vp?

[piko-piko]

--keep-zoom-vp keeps the viewport zoom value AND the viewport offset value. In my manga reader use case, i want to keep the viewport zoom value, but use the offset value set by the --offset for the images loaded by next-img, so when i "turn the page" i'm back at top of the page (--offset +0+0) with my vp, but with the zoom value kept as it is in the viewport

Probably a less confusing way of implementing this would be to have
--keep-zoom-vp for keeping just the vp zoom
and
--keep-offset-vp for keeping just the vp offset
but i didn't want to break selfishly the way --keep-zoom-vp works currently