Introduce WarheadArgs

- Passes additional arguments to warheads on impact
- Uses that to reduce parameter count of DoImpact by 1

OpenRA.Mods.Common/Projectiles/AreaBeam.cs

@@ -11,6 +11,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using OpenRA.GameRules;
 using OpenRA.Graphics;
 using OpenRA.Mods.Common.Graphics;
@@ -231,7 +232,13 @@ namespace OpenRA.Mods.Common.Projectiles
 				foreach (var a in actors)
 				{
 					var adjustedModifiers = args.DamageModifiers.Append(GetFalloff((args.Source - a.CenterPosition).Length));
-					args.Weapon.Impact(Target.FromActor(a), args.SourceActor, adjustedModifiers);
+
+					var warheadArgs = new WarheadArgs(args)
+					{
+						DamageModifiers = adjustedModifiers.ToArray(),
+					};
+
+					args.Weapon.Impact(Target.FromActor(a), warheadArgs);
 				}
 			}