[Therion] Depth adds on surface if it is in a different sub-survey

[Tarquin Wilton-Jones]

Hi folks,

Ages ago, I filed https://github.com/therion/therion/issues/165 and
Stacho fixed it (thanks!).

We have just stumbled on another variant of it in a survey where both
surface and the cave are rendered in a map (a few slopes on the
surface), but the surface comes from a different sub-survey. This causes
Therion to add on the depth of the surface legs too, so the cave gets
far deeper than it should be.

https://github.com/therion/therion/issues/392

I wonder how many cave surveys there are out there with the wrong depth
because of this, since I would think that the vast majority of big
surveys are done this way...

o_O

Tarquin