GPGX - keep a savestate buffer for SaveStateBinary, yeah maybe it shoudln't be every cores responsibility, but until it doesn't, this greatly improves rewind and tastudio performance

src/BizHawk.Emulation.Cores/Consoles/Sega/gpgx64/GPGX.IStatable.cs

@@ -5,6 +5,8 @@ namespace BizHawk.Emulation.Cores.Consoles.Sega.gpgx
 {
 	public partial class GPGX : IStatable
 	{
+		private byte[] _stateBuffer;
+
 		public void LoadStateBinary(BinaryReader reader)
 		{
 			_elf.LoadStateBinary(reader);
@@ -37,12 +39,23 @@ namespace BizHawk.Emulation.Cores.Consoles.Sega.gpgx
 
 		public byte[] SaveStateBinary()
 		{
+			if (_stateBuffer != null)
+			{
+				using var stream = new MemoryStream(_stateBuffer);
+				using var writer = new BinaryWriter(stream);
+				SaveStateBinary(writer);
+				writer.Flush();
+				writer.Close();
+				return _stateBuffer;
+			}
+
 			using var ms = new MemoryStream();
 			using var bw = new BinaryWriter(ms);
 			SaveStateBinary(bw);
 			bw.Flush();
-			ms.Close();
-			return ms.ToArray();
+			_stateBuffer = ms.ToArray();
+			bw.Close();
+			return _stateBuffer;
 		}
 	}
 }