Feature request: Allow specifying an offset

[bodby]

This would be easier than cropping a wallpaper for different aspect resolutions/make it easier to center a part of the wallpaper.

I feel like this would work for all modes except for stretch and tile.

[mstoeckl]

I currently am not sure whether this would be a good thing to add. swaybg tends to be minimal† and there are a number of more-featured alternatives or wrapper programs to which a feature like this would be better suited. (Some are listed at https://github.com/swaywm/sway/wiki/Useful-add-ons-for-sway#wallpaper; I haven't looked into them all, but mpvpaper can be used draw static images with offset if you apply flags like -o "pause video-pan-x=0.5".).

† In part because, to maintain consistency between sway, swaybg, and swaylock's background configuration, any new options would eventually lead to modifications to all three, making features more expensive to maintain than one might expect...