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/SpriteLoader.cs

@@ -9,10 +9,7 @@
  */
 #endregion
 
-using System;
-using System.Collections.Generic;
 using System.IO;
-using System.Linq;
 using OpenRA.FileSystem;
 using OpenRA.Primitives;
 
@@ -67,94 +64,6 @@ namespace OpenRA.Graphics
 		bool DisableExportPadding { get; }
 	}
 
-	public class SpriteCache
-	{
-		public readonly Cache<SheetType, SheetBuilder> SheetBuilders;
-		readonly ISpriteLoader[] loaders;
-		readonly IReadOnlyFileSystem fileSystem;
-
-		readonly Dictionary<string, List<Sprite[]>> sprites = new Dictionary<string, List<Sprite[]>>();
-		readonly Dictionary<string, ISpriteFrame[]> unloadedFrames = new Dictionary<string, ISpriteFrame[]>();
-		readonly Dictionary<string, TypeDictionary> metadata = new Dictionary<string, TypeDictionary>();
-
-		public SpriteCache(IReadOnlyFileSystem fileSystem, ISpriteLoader[] loaders)
-		{
-			SheetBuilders = new Cache<SheetType, SheetBuilder>(t => new SheetBuilder(t));
-
-			this.fileSystem = fileSystem;
-			this.loaders = loaders;
-		}
-
-		/// <summary>
-		/// Returns the first set of sprites with the given filename.
-		/// If getUsedFrames is defined then the indices returned by the function call
-		/// are guaranteed to be loaded.  The value of other indices in the returned
-		/// array are undefined and should never be accessed.
-		/// </summary>
-		public Sprite[] this[string filename, Func<int, IEnumerable<int>> getUsedFrames = null]
-		{
-			get
-			{
-				var allSprites = sprites.GetOrAdd(filename);
-				var sprite = allSprites.FirstOrDefault();
-
-				if (!unloadedFrames.TryGetValue(filename, out var unloaded))
-					unloaded = null;
-
-				// This is the first time that the file has been requested
-				// Load all of the frames into the unused buffer and initialize
-				// the loaded cache (initially empty)
-				if (sprite == null)
-				{
-					unloaded = FrameLoader.GetFrames(fileSystem, filename, loaders, out var fileMetadata);
-					unloadedFrames[filename] = unloaded;
-					metadata[filename] = fileMetadata;
-
-					sprite = new Sprite[unloaded.Length];
-					allSprites.Add(sprite);
-				}
-
-				// HACK: The sequence code relies on side-effects from getUsedFrames
-				var indices = getUsedFrames != null ? getUsedFrames(sprite.Length) :
-					Enumerable.Range(0, sprite.Length);
-
-				// Load any unused frames into the SheetBuilder
-				if (unloaded != null)
-				{
-					foreach (var i in indices)
-					{
-						if (unloaded[i] != null)
-						{
-							sprite[i] = SheetBuilders[SheetBuilder.FrameTypeToSheetType(unloaded[i].Type)].Add(unloaded[i]);
-							unloaded[i] = null;
-						}
-					}
-
-					// All frames have been loaded
-					if (unloaded.All(f => f == null))
-						unloadedFrames.Remove(filename);
-				}
-
-				return sprite;
-			}
-		}
-
-		/// <summary>
-		/// Returns a TypeDictionary containing any metadata defined by the frame
-		/// or null if the frame does not define metadata.
-		/// </summary>
-		public TypeDictionary FrameMetadata(string filename)
-		{
-			if (!metadata.TryGetValue(filename, out var fileMetadata))
-			{
-				FrameLoader.GetFrames(fileSystem, filename, loaders, out fileMetadata);
-				metadata[filename] = fileMetadata;
-			}
-
-			return fileMetadata;
-		}
-	}
-
 	public class FrameCache
 	{
 		readonly Cache<string, ISpriteFrame[]> frames;