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

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