Change indexes into tile arrays to be stored as bytes to save memory.

The index value needs only be big enough to handle all defined terrain types. This is a low number so we can save memory by defining it as a byte. This particularly saves memory for the CustomTerrain field in the Map class, which defines a cell layer for the map using tile indexes, so we can reduce the size of that layer 4x as a result.

OpenRA.Game/Graphics/Minimap.cs

@@ -115,7 +115,7 @@ namespace OpenRA.Graphics
 						var mapX = x + b.Left;
 						var mapY = y + b.Top;
 						var custom = map.CustomTerrain[mapX, mapY];
-						if (custom == -1)
+						if (custom == byte.MaxValue)
 							continue;
 						colors[y * stride + x] = world.TileSet[custom].Color.ToArgb();
 					}