lazy-initialize zstd contexts to reduce alloations

This becomes mostly apparent in the PlayMovie dialog where one zstd instance is created for every single movie, churning significant amounts of memory even though zstd isn't even used

src/BizHawk.Emulation.Common/zstd/Zstd.cs

@@ -1,13 +1,13 @@
-using System;
-using System.IO;
-using System.Runtime.InteropServices;
-
-using BizHawk.BizInvoke;
-using BizHawk.Common;
-
-namespace BizHawk.Emulation.Common
-{
-	public sealed class Zstd : IDisposable
+using System;
+using System.IO;
+using System.Runtime.InteropServices;
+
+using BizHawk.BizInvoke;
+using BizHawk.Common;
+
+namespace BizHawk.Emulation.Common
+{
+	public sealed class Zstd : IDisposable
 	{
 		private sealed class ZstdCompressionStreamContext : IDisposable
 		{
@@ -380,14 +380,8 @@ namespace BizHawk.Emulation.Common
 			MaxCompressionLevel = _lib.ZSTD_maxCLevel();
 		}
 
-		private readonly ZstdCompressionStreamContext _compressionStreamContext;
-		private readonly ZstdDecompressionStreamContext _decompressionStreamContext;
-
-		public Zstd()
-		{
-			_compressionStreamContext = new();
-			_decompressionStreamContext = new();
-		}
+		private ZstdCompressionStreamContext? _compressionStreamContext;
+		private ZstdDecompressionStreamContext? _decompressionStreamContext;
 
 		private bool _disposed = false;
 
@@ -395,8 +389,8 @@ namespace BizHawk.Emulation.Common
 		{
 			if (!_disposed)
 			{
-				_compressionStreamContext.Dispose();
-				_decompressionStreamContext.Dispose();
+				_compressionStreamContext?.Dispose();
+				_decompressionStreamContext?.Dispose();
 				_disposed = true;
 			}
 		}
@@ -429,6 +423,7 @@ namespace BizHawk.Emulation.Common
 				throw new ArgumentOutOfRangeException(nameof(compressionLevel));
 			}
 
+			_compressionStreamContext ??= new();
 			_compressionStreamContext.InitContext(compressionLevel);
 			return new ZstdCompressionStream(stream, _compressionStreamContext);
 		}
@@ -446,6 +441,7 @@ namespace BizHawk.Emulation.Common
 		/// <returns>zstd decompression stream</returns>
 		public Stream CreateZstdDecompressionStream(Stream stream)
 		{
+			_decompressionStreamContext ??= new();
 			_decompressionStreamContext.InitContext();
 			return new ZstdDecompressionStream(stream, _decompressionStreamContext);
 		}
@@ -480,5 +476,5 @@ namespace BizHawk.Emulation.Common
 			dstream.CopyTo(ret);
 			return ret;
 		}
-	}
-}
+	}
+}