Refactoring pass.

- Extract an enum for edges rather than using magic numbers for everything.
- Remove duplicated code between FoggedEdges and ShroudedEdges by hosting the visibility function into a delegate.
- Make minimap methods more readable.
- Tidy formatting.
- Make some fields readonly.
- Remove unused usings.

OpenRA.Game/Map/CellRegion.cs

@@ -8,11 +8,8 @@
  */
 #endregion
 
-using System;
 using System.Collections;
 using System.Collections.Generic;
-using System.Drawing;
-using OpenRA.Graphics;
 
 namespace OpenRA
 {