nitrogen tissue saturation toggle is broken

[Salvo Tomaselli]

Hello,

Just writing to point out that the toggle to show/hide the nitrogen
saturation of tissues is broken.

I had it shown, then pressed it, and now it is forever hidden.

However, if I also show the calculated ceiling, it shows the tissue
saturation too, but removes it again if I hide the calculated ceiling.

Probably something very trivial to fix, but it's too hard now to
compile subsurface so this email doesn't come with a patch :P

Best!