Tweaks for map generator performance.

The map generator is already performant, but we can remove a few rough edges from infrastructure it interacts with.

- Add TypeDictionary constructor to clone from an existing copy. The map editor infrastructure relies heavily on cloning ActorReferences and the underlying dictionaries. A dedicated clone method avoids reflection overhead for CreateTypeContainer on a fresh instance.
- Use ThrowIndexOutOfRangeException helper. This allows the Index method to be inlined.
- Reuse lists in FloodFill. This avoids allocating and resizing new lists on each loop when we can reuse the existing capacity.
- Manage CellEntryChanged at the layer rather than individual previews. We only need to attach/detach from the delegate once at the layer, rather than many times as previews are added and removed.

OpenRA.Mods.Common/Traits/ProductionFromMapEdge.cs

@@ -82,13 +82,12 @@ namespace OpenRA.Mods.Common.Traits
 
 			self.World.AddFrameEndTask(w =>
 			{
-				var td = new TypeDictionary();
-				foreach (var init in inits)
-					td.Add(init);
-
-				td.Add(new LocationInit(location.Value));
-				td.Add(new CenterPositionInit(pos));
-				td.Add(new FacingInit(initialFacing));
+				var td = new TypeDictionary(inits)
+				{
+					new LocationInit(location.Value),
+					new CenterPositionInit(pos),
+					new FacingInit(initialFacing)
+				};
 
 				var newUnit = self.World.CreateActor(producee.Name, td);