Rewrite sequence loading logic.

Multiple layers of Lazy<T>ness are replaced with
an explicit two-part loading scheme.

Sequences are parsed immediately, without the need
for the sprite assets, and tell the SpriteCache
which frames they need. Use-cases that want the
actual sprites can then tell the SpriteCache to
load the frames and the sequences to resolve the
sprites.

OpenRA.Game/Graphics/Animation.cs

@@ -22,7 +22,7 @@ namespace OpenRA.Graphics
 		public string Name { get; private set; }
 		public bool IsDecoration { get; set; }
 
-		readonly SequenceProvider sequences;
+		readonly SequenceSet sequences;
 		readonly Func<WAngle> facingFunc;
 		readonly Func<bool> paused;