Checked LINQ queries and collections for inefficiencies.

- Made Array.IndexOf available via extension method.
- Made ToHashSet extension method.
- Change collections queried often via Contains into sets.
- Avoid Count() extension if Count or Length property exist.
- Made Count() > 0 checks and variations calls to Any() instead.
- Don't call ToList/ToArray if there is no benefit to materializing the sequence.
- If the sequence does benefit from materialization, follow this general pattern:
  - Collection queried often via Contains use ToHashSet to speed up lookups.
  - Short lived variables use ToList. This is because ToArray requires an extra copy to output the final size.
  - Collections persisted into fields or for a long time use ToArray to minimize memory overhead.

OpenRA.Game/Graphics/WorldRenderer.cs

@@ -80,12 +80,11 @@ namespace OpenRA.Graphics
 		List<IRenderable> GenerateRenderables()
 		{
 			var actors = World.ScreenMap.ActorsInBox(Viewport.TopLeft, Viewport.BottomRight)
-				.Append(World.WorldActor)
-				.ToList();
+				.Append(World.WorldActor);
 
 			// Include player actor for the rendered player
 			if (World.RenderPlayer != null)
-				actors.Add(World.RenderPlayer.PlayerActor);
+				actors = actors.Append(World.RenderPlayer.PlayerActor);
 
 			var worldRenderables = actors.SelectMany(a => a.Render(this));
 			if (World.OrderGenerator != null)