Add skirts to tile boundaries

[rmarquis]

The Martini algorithm create some visible hole in the mesh, especially visible in mountain area. A common solution to that issue is to create "skirts" around the tiles, like a vertical wall from the surface to some defined value underground to fill these holes.

Ideally:

• one more colored pixel of each tile should be taken into account so the skirts can use that color information for better blending
• normals should stay unchanged at the border limit to ensure light effect stays correct.