Issue title: Splatting planets? - Identify land types for passing to shader.
 Issue body: I was thinking on writing splatting shaders for the planets, in order to do so I must have a way to identify different land types like, rock, snow, grass, ice, lava, maybe agriculture land (for those vast acres), and cityland (road networks and similar). 

Identifying these would be great for writing splatter shaders, I was thinking maybe just use shader preprosessors or uniforms for different land types. Then texture them accordingly. If I were to have these landtypes sendt in `GeoSphereSurfaceMaterial38d1:d2b4:3d06:83e8:96a5:8c2:893d:aba5SetGSUniforms()` I'd be a start. But it also possible to use slope/height textureing for splatter, but that would require to "theme" the planet in some way for choosing splatter textures.

These are just ideas in my head.

 Comments: 
 Comment 0: It might be an idea to take this to the dev mailing list, at least until it becomes a little more concrete.

 Comment 1: Yeah I will, thanks.

{'source': 'GithubIssues', 'repo_name_with_owner': 'pioneerspacesim/pioneer', 'issue_id': 8899587, 'repo_stars': 1237.0}