Fix editor grid overlay height coloring calculation

The change fixes two issues with the map editor's height-based grid
overlay color calculation:

- If the height exceeded the end of the color scale, the engine would
  crash due to attempting an out of bounds read. Now, the calculation
  caps out to using the last color in the scale.
- The calculation assumed the height step was always 512, even for
  RectangularIsometric maps. Now, the (halved) TileScale of the map's
  grid type is used.

OpenRA.Mods.Common/Traits/World/TerrainGeometryOverlay.cs

@@ -9,6 +9,7 @@
  */
 #endregion
 
+using System;
 using System.Collections.Generic;
 using OpenRA.Graphics;
 using OpenRA.Mods.Common.Commands;
@@ -57,6 +58,8 @@ namespace OpenRA.Mods.Common.Traits
 
 			var map = wr.World.Map;
 			var colors = wr.World.Map.Rules.TerrainInfo.HeightDebugColors;
+			var lastColor = colors.Length - 1;
+			var heightStep = map.Grid.TileScale / 2;
 			var mouseCell = wr.Viewport.ViewToWorld(Viewport.LastMousePos).ToMPos(wr.World.Map);
 
 			foreach (var uv in wr.Viewport.AllVisibleCells.CandidateMapCoords)
@@ -77,8 +80,8 @@ namespace OpenRA.Mods.Common.Traits
 						var j = (i + 1) % p.Length;
 						var start = pos + p[i];
 						var end = pos + p[j];
-						var startColor = colors[height + p[i].Z / 512];
-						var endColor = colors[height + p[j].Z / 512];
+						var startColor = colors[Math.Min(lastColor, height + p[i].Z / heightStep)];
+						var endColor = colors[Math.Min(lastColor, height + p[j].Z / heightStep)];
 						yield return new LineAnnotationRenderable(start, end, width, startColor, endColor);
 					}
 				}