add an sgmodel conversion of the Lanner

Incomplete. Missing things:

* Patterns.
* Label positions.
* Decal positions.
* Hydraulic rams on the undercarriage.
* Collision warning lights
* Headlights (though I've been told we don't want those)
make the Lanner ship and Intro spinner use the sgmodel version
delete the LMR version of the Lanner
lanner: fix errors with vertex normals (fixes edge glitches)
lanner: add a pattern texture for the coloured stripe
lanner: remove duplicated vertices in collision mesh
lanner: add a medium quality (single batch) LOD version
lanner: reduce pattern texture size to 256x256
lanner: remove alpha channel on diffuse texture
lanner: switch pattern texture to grayscale
Merge remote-tracking branch 'johnbartholomew/sgmodel-lanner'