Add terrain surface visualisation

[Wookey]

We currently have surface data as legs, which can be used to make a terrain grid (using terraintool to import DEM data). A proper surface using openGL would make for much better visualisations, separate surface survey/GPS data from terrain models and allow later additton of map-draping and similar features.

Related features are data reading, a DEM survex data type, DEM import, co-ordinate conversion, and overlay images, but those are subjects for separate tickets.

Some work on this has been done in 2002 and 2011, as discussed in various 'Surface data' threads at: ​http://lists.survex.com/pipermail/survex/2011-September/thread.html Since then the survex VCS has been converted from SVN to git so the links are broken.

The important bits are these commits: r2035 8ed7e813da6bb872ef3d5d5f7064c3ad16ecb921 ​https://gitorious.org/survex/survex/commit/8ed7e813da6bb872ef3d5d5f7064c3ad16ecb921 r2059 33b2094393ec862b7206d12f49c9735a53d8fb88 ​https://gitorious.org/survex/survex/commit/33b2094393ec862b7206d12f49c9735a53d8fb88

And this is Stuart's hacky patch showing that the core if this is straightforward:

[Wookey]

Hacky openGL surface visualisation (by Stuart Bennet and Wookey)

[Olly Betts]

Note that you don't want to do the rendering with QUADS - those are only generally suitable for quadrilaterals which lie exactly in a plane, but for terrain they rarely will. If you can see both sides at once, then you'll likely get visual glitches, and it's quite likely you'll be able to if you look at the edge of the terrain from a suitable angle.

We shouldn't really be using QUADS for the tubes either (and there's a FIXME to that effect), but it's harder to get a view such that you see a glitch for those.