Add support on-demand sprite frame loading.

Passing the getUsedFrames argument will cause frames to be buffered
locally (and not added to the SheetBuilder) until they are explicitly
requested. This allows unused frames in sprite files to be skipped
instead of wasting space in GPU memory.

OpenRA.Game/Graphics/SpriteLoader.cs

@@ -9,6 +9,7 @@
  */
 #endregion
 
+using System;
 using System.Collections.Generic;
 using System.Drawing;
 using System.IO;
@@ -48,6 +49,7 @@ namespace OpenRA.Graphics
 		readonly IReadOnlyFileSystem fileSystem;
 
 		readonly Dictionary<string, List<Sprite[]>> sprites = new Dictionary<string, List<Sprite[]>>();
+		readonly Dictionary<string, ISpriteFrame[]> unloadedFrames = new Dictionary<string, ISpriteFrame[]>();
 
 		public SpriteCache(IReadOnlyFileSystem fileSystem, ISpriteLoader[] loaders, SheetBuilder sheetBuilder)
 		{
@@ -56,21 +58,56 @@ namespace OpenRA.Graphics
 			this.loaders = loaders;
 		}
 
-		Sprite[] LoadSprite(string filename, List<Sprite[]> cache)
-		{
-			var sprite = SpriteLoader.GetSprites(fileSystem, filename, loaders, SheetBuilder);
-			cache.Add(sprite);
-			return sprite;
-		}
-
-		/// <summary>Returns the first set of sprites with the given filename.</summary>
-		public Sprite[] this[string filename]
+		/// <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();
-				return sprite ?? LoadSprite(filename, allSprites);
+
+				ISpriteFrame[] unloaded;
+				if (!unloadedFrames.TryGetValue(filename, out 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);
+					unloadedFrames[filename] = unloaded;
+
+					sprite = new Sprite[unloaded.Length];
+					allSprites.Add(sprite);
+				}
+
+				// Load any unused frames into the SheetBuilder
+				if (unloaded != null)
+				{
+					var indices = getUsedFrames != null ? getUsedFrames(sprite.Length) :
+						Enumerable.Range(0, sprite.Length);
+
+					foreach (var i in indices)
+					{
+						if (unloaded[i] != null)
+						{
+							sprite[i] = SheetBuilder.Add(unloaded[i]);
+							unloaded[i] = null;
+						}
+					}
+
+					// All frames have been loaded
+					if (unloaded.All(f => f == null))
+						unloadedFrames.Remove(filename);
+				}
+
+				return sprite;
 			}
 		}
 
@@ -83,7 +120,12 @@ namespace OpenRA.Graphics
 		/// <summary>Loads and caches a new instance of sprites with the given filename</summary>
 		public Sprite[] Reload(string filename)
 		{
-			return LoadSprite(filename, sprites.GetOrAdd(filename));
+			var sprite = FrameLoader.GetFrames(fileSystem, filename, loaders)
+				.Select(a => SheetBuilder.Add(a))
+				.ToArray();
+
+			sprites.GetOrAdd(filename).Add(sprite);
+			return sprite;
 		}
 	}
 
@@ -93,19 +135,14 @@ namespace OpenRA.Graphics
 
 		public FrameCache(IReadOnlyFileSystem fileSystem, ISpriteLoader[] loaders)
 		{
-			frames = new Cache<string, ISpriteFrame[]>(filename => SpriteLoader.GetFrames(fileSystem, filename, loaders));
+			frames = new Cache<string, ISpriteFrame[]>(filename => FrameLoader.GetFrames(fileSystem, filename, loaders));
 		}
 
 		public ISpriteFrame[] this[string filename] { get { return frames[filename]; } }
 	}
 
-	public static class SpriteLoader
+	public static class FrameLoader
 	{
-		public static Sprite[] GetSprites(IReadOnlyFileSystem fileSystem, string filename, ISpriteLoader[] loaders, SheetBuilder sheetBuilder)
-		{
-			return GetFrames(fileSystem, filename, loaders).Select(a => sheetBuilder.Add(a)).ToArray();
-		}
-
 		public static ISpriteFrame[] GetFrames(IReadOnlyFileSystem fileSystem, string filename, ISpriteLoader[] loaders)
 		{
 			using (var stream = fileSystem.Open(filename))