Make HitShape mandatory for damaging actors and refactor warheads.

* Adds support for linking Armor traits to HitShapes.
* Adds spread support to TargetDamageWarhead
* Removes ring-damage support from HealthPercentageDamage
* Removes IsValidAgainst check from DoImpact(Actor victim...) overload
  and instead lets warheads perform the check beforehand
  (to avoid HitShape look-ups on invalid targets).
* Reduces duplication and improves readability of Warhead implementations

OpenRA.Mods.Common/UpdateRules/Rules/20180923/RemoveHealthPercentageRing.cs

@@ -0,0 +1,45 @@
+#region Copyright & License Information
+/*
+ * Copyright 2007-2018 The OpenRA Developers (see AUTHORS)
+ * This file is part of OpenRA, which is free software. It is made
+ * available to you under the terms of the GNU General Public License
+ * as published by the Free Software Foundation, either version 3 of
+ * the License, or (at your option) any later version. For more
+ * information, see COPYING.
+ */
+#endregion
+
+using System.Collections.Generic;
+
+namespace OpenRA.Mods.Common.UpdateRules.Rules
+{
+	public class RemoveHealthPercentageRing : UpdateRule
+	{
+		public override string Name { get { return "Remove ring support from HealthPercentageDamage warheads' Spread"; } }
+		public override string Description
+		{
+			get
+			{
+				return "Setting a second value in this warheads' Spread to define a 'ring' is no longer supported.";
+			}
+		}
+
+		public override IEnumerable<string> UpdateWeaponNode(ModData modData, MiniYamlNode weaponNode)
+		{
+			foreach (var node in weaponNode.ChildrenMatching("Warhead"))
+			{
+				if (node.NodeValue<string>() == "HealthPercentageDamage")
+				{
+					foreach (var spreadNode in node.ChildrenMatching("Spread"))
+					{
+						var oldValue = spreadNode.NodeValue<string[]>();
+						if (oldValue.Length > 1)
+							spreadNode.ReplaceValue(oldValue[0]);
+					}
+				}
+			}
+
+			yield break;
+		}
+	}
+}