Add terrain coloration

[amterp]

From the spec:

• The colour of the terrain must correspond in a sensible way with the height of the terrain at any particular point (for example rocky outcrops or snow on top of mountains and grass or soil in valleys).

Here are the levels I am thinking of:

• Sand (near water).
• Grass (higher than sand).
• Darkish grey rocks (for mountains, higher than grass).
• White snow (for mountain peaks when they're particularly high, higher than grey rocks).