Add initial prototype.

4-high/gel/source/terrain/gel-terrain.ads

@@ -0,0 +1,16 @@
+with
+     gel.Sprite,
+     gel.World;
+
+package gel.Terrain
+--
+--  Provides a constructor for heightmap terrain.
+--
+is
+
+   function new_Terrain (World        : in gel.World.view;
+                         heights_File : in String;
+                         texture_File : in String        := "";
+                         Scale        : in math.Vector_3 := (1.0, 1.0, 1.0)) return access gel.Sprite.Grid;
+
+end gel.Terrain;