Create a separate FrameCache for caching sprite frames.

We split the caching SpriteLoader into a SpriteCache and FrameCache. SpriteLoader instead becomes a holder for static loading methods.

Only a few classes loaded sprite frames, and they all use it with a transient cache. By moving this method into a new class, we can lose the now redundant frame cache, saving on memory significantly since the frame data array can be reclaimed by the GC. This saves ~58 MiB on frames and ~4 MiB on the caching dictionary in simple tests.

OpenRA.Mods.Common/UtilityCommands/ConvertSpriteToPngCommand.cs

@@ -41,8 +41,7 @@ namespace OpenRA.Mods.Common.UtilityCommands
 
 			var palette = new ImmutablePalette(args[2], shadowIndex);
 
-			var frames = new SpriteLoader(modData.SpriteLoaders, new string[0], null)
-				.LoadAllFrames(src);
+			var frames = SpriteLoader.GetFrames(src, new string[0], modData.SpriteLoaders);
 
 			var usePadding = !args.Contains("--nopadding");
 			var count = 0;