Correctly calculate TerrainFace seams

[Hoimar]

Currently when TerrainFace meshes meet, they only match up when they have the same resolution, as the same noise value will be sampled for all vertices in the same position. When different LOD levels meet however, between two vertices in the coarser mesh there will be an extra vertice in the finer mesh (see screenshot) which samples a different value.

The first solution I'll try is to use the median between the left and right vertice for every odd edge vertice in the finer mesh to align to the coarse mesh. This way I won't have to deal with creating additional geometry and/or stitching the meshes (for now at least, will see how this works out).