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.Mods.Common/UtilityCommands/ConvertSpriteToPngCommand.cs

@@ -47,7 +47,7 @@ namespace OpenRA.Mods.Common.UtilityCommands
 
 			var palette = new ImmutablePalette(args[2], shadowIndex);
 
-			var frames = SpriteLoader.GetFrames(File.OpenRead(src), modData.SpriteLoaders);
+			var frames = FrameLoader.GetFrames(File.OpenRead(src), modData.SpriteLoaders);
 
 			var usePadding = !args.Contains("--nopadding");
 			var count = 0;