Fix bathypelagic background image artifact (horizontal line)

[hhyyrylainen]

The backgrounds have very slight artifacts (horizontal line here):

Probably something to do with the shader / layer texture repeats.

[hhyyrylainen]

This is probably fixed as I've not seen this for multiple years. If anyone sees this still happening please reopen / comment on this.

[Oqwert]

Still happening in the bathypelagic background, patch 0.7.1 (right above my cell)

[hhyyrylainen]

Zooming in I can see the line in the screenshot. I've reopened this issue.

I think the issue is now more likely that that one specific texture has a slight problem with a non-continuity region around an edge of it. So an artist would be required to ever so slightly edit that one edge of the texture to no longer cause that line when the texture wraps.