Activity, fixes.

Do not call SkipDoneActivities method recursively via the
NextActivity property. Rather use the nextActivity member.
Avoiding additional function calls and a recursively
growing stack.

Do not call ChildActivity and NextActivity properties
twice in a row. Once to test for null and after to access
it's value. It will cause the complete list of activities
to be traversed twice looking for non done activities.

Replace Queue method with a version that does not the
NextActivity property causing an extra call to
SkipDoneActivities. Avoid calling Queue recursively.

Similar replace QueueChild with a version that does
not call additional methods.

Note that ActivitiesImplementing returns only non
done activities. The method name does not suggest this.

Please consider making NextActivity a method to cleary indicate it
involves the logic of skipping Done activities. To let
the called know it is 'expensive'.

Please consider renaming the protected property ChildActivity to
FirstChildActivityNotDone to avoid it being used as childActivity.

Please consider maintaining a pointer to the first
non done activity. This avoids the need the each time find it.

OpenRA.Game/Activities/Activity.cs

@@ -74,7 +74,7 @@ internal static Activity SkipDoneActivities(Activity first)
 			// drop valid activities queued after it. Walk the queue until we find a valid activity or
 			// (more likely) run out of activities.
 			while (first != null && first.State == ActivityState.Done)
-				first = first.NextActivity;
+				first = first.nextActivity;
 
 			return first;
 		}
@@ -120,7 +120,8 @@ public Activity TickOuter(Actor self)
 				lastRun = Tick(self);
 
 			// Avoid a single tick delay if the childactivity was just queued.
-			if (ChildActivity != null && ChildActivity.State == ActivityState.Queued)
+			var ca = ChildActivity;
+			if (ca != null && ca.State == ActivityState.Queued)
 			{
 				if (ChildHasPriority)
 					lastRun = TickChild(self) && finishing;
@@ -206,18 +207,18 @@ public virtual void Cancel(Actor self, bool keepQueue = false)
 
 		public void Queue(Activity activity)
 		{
-			if (NextActivity != null)
-				NextActivity.Queue(activity);
-			else
-				NextActivity = activity;
+			var it = this;
+			while (it.nextActivity != null)
+				it = it.nextActivity;
+			it.nextActivity = activity;
 		}
 
 		public void QueueChild(Activity activity)
 		{
-			if (ChildActivity != null)
-				ChildActivity.Queue(activity);
+			if (childActivity != null)
+				childActivity.Queue(activity);
 			else
-				ChildActivity = activity;
+				childActivity = activity;
 		}
 
 		/// <summary>
@@ -269,15 +270,21 @@ public IEnumerable<string> DebugLabelComponents()
 
 		public IEnumerable<T> ActivitiesImplementing<T>(bool includeChildren = true) where T : IActivityInterface
 		{
-			if (includeChildren && ChildActivity != null)
-				foreach (var a in ChildActivity.ActivitiesImplementing<T>())
-					yield return a;
+			// Skips Done child and next activities
+			if (includeChildren)
+			{
+				var ca = ChildActivity;
+				if (ca != null)
+					foreach (var a in ca.ActivitiesImplementing<T>())
+						yield return a;
+			}
 
 			if (this is T)
 				yield return (T)(object)this;
 
-			if (NextActivity != null)
-				foreach (var a in NextActivity.ActivitiesImplementing<T>())
+			var na = NextActivity;
+			if (na != null)
+				foreach (var a in na.ActivitiesImplementing<T>())
 					yield return a;
 		}
 	}