Introduce WarheadArgs

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

OpenRA.Mods.Common/Projectiles/Railgun.cs

@@ -188,12 +188,12 @@ namespace OpenRA.Mods.Common.Projectiles
 					animationComplete = true;
 
 				if (!info.DamageActorsInLine)
-					args.Weapon.Impact(Target.FromPos(target), args.SourceActor, args.DamageModifiers);
+					args.Weapon.Impact(Target.FromPos(target), new WarheadArgs(args));
 				else
 				{
 					var actors = world.FindActorsOnLine(args.Source, target, info.BeamWidth);
 					foreach (var a in actors)
-						args.Weapon.Impact(Target.FromActor(a), args.SourceActor, args.DamageModifiers);
+						args.Weapon.Impact(Target.FromActor(a), new WarheadArgs(args));
 				}
 			}