Rewrite sequence linting / missing file handling.

- Distinguish between missing sequences and missing sprites
- Lint default sequences as well as maps
- Improved performance
- Correctly handle null images

OpenRA.Game/ModData.cs

@@ -79,7 +79,6 @@ namespace OpenRA
 				throw new InvalidOperationException("Unable to find a sequence loader for type '{0}'.".F(sequenceFormat.Type));
 
 			SpriteSequenceLoader = (ISpriteSequenceLoader)sequenceCtor.Invoke(new[] { this });
-			SpriteSequenceLoader.OnMissingSpriteError = s => Log.Write("debug", s);
 
 			var modelFormat = Manifest.Get<ModelSequenceFormat>();
 			var modelLoader = ObjectCreator.FindType(modelFormat.Type + "Loader");