Rework minefield visualisation.

OpenRA.Game/Activities/Activity.cs

@@ -12,6 +12,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using OpenRA.Graphics;
 using OpenRA.Primitives;
 using OpenRA.Traits;
 
@@ -23,13 +24,15 @@ namespace OpenRA.Activities
 	{
 		public readonly Target Target;
 		public readonly Color Color;
+		public readonly Sprite Tile;
 
-		public TargetLineNode(Target target, Color color)
+		public TargetLineNode(Target target, Color color, Sprite tile = null)
 		{
 			// Note: Not all activities are drawable. In that case, pass Target.Invalid as target,
 			// if "yield break" in TargetLineNode(Actor self) is not feasible.
 			Target = target;
 			Color = color;
+			Tile = tile;
 		}
 	}