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/Production.cs

@@ -54,9 +54,7 @@ namespace OpenRA.Mods.Common.Traits
 			var exitLocations = new List<CPos>();
 
 			// Clone the initializer dictionary for the new actor
-			var td = new TypeDictionary();
-			foreach (var init in inits)
-				td.Add(init);
+			var td = new TypeDictionary(inits);
 
 			if (exitinfo != null && self.OccupiesSpace != null && producee.HasTraitInfo<IOccupySpaceInfo>())
 			{