[Therion] More show and hide

[Stacho Mudrak]

> Have just figured out that the following statement will hide morphed
> sketches of caves.
>
>    symbol-hide group surface-centreline  # hides morphed sketches as well??
>
> Is this by design, or is it a side effect of something else I may be doing?
>
> I'm using version 5.2
>
> Perhaps it is because it is drawn on the same layer as the surface image?

Well, currently the background sketches are drawn as surface images -
so surface top|bottom|off should affect it, but I am not sure if
hide/show surface-centerline will. I need to check this. It is planned
to be placed into same PDF layer as scraps are, but it will take some
time, until it will be implemented. Do you have some problem with it?

Regards, S.