make FFmpegService more reliable (fixes inexplicable hangs in reading ffmpeg output which breaks discohawk audio extractor sometimes and probably other things sometimes unpredictably). fixes #2952

src/BizHawk.Common/FFmpegService.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Threading;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
@@ -79,9 +80,53 @@ namespace BizHawk.Common
 					RedirectStandardError = true
 				};
 
+		string result = "";
+
+		//probably naturally thread-safe...
+		//make a lock if not
+		Action<StreamReader,bool> readerloop = (reader,ignore) =>
+		{
+			string line = "";
+			int idx = 0;
+			for (; ; )
+			{
+				int c = reader.Read();
+				if (c == -1) 
+					break;
+				else if (c == '\r')
+					idx = 0;
+				else if (c == '\n')
+				{
+					if(!ignore)
+						lock(oInfo)
+							result += line + "\n";
+					line = "";
+					idx = 0;
+				}
+				else
+				{
+					if (idx < line.Length)
+						line = line.Substring(0, idx) + (char)c + line.Substring(idx + 1);
+					else
+						line += (char)c;
+					idx++;
+				}
+			}
+
+			if(!ignore)
+				if(line != "")
+					lock(oInfo)
+						result += line + "\n"; //not sure about the final \n but i concat it after each finished line so why not.. whatever
+		};
+
 			Process proc = Process.Start(oInfo);
-			string result = proc.StandardOutput.ReadToEnd();
-			result += proc.StandardError.ReadToEnd();
+
+			var tout = new Thread(() => readerloop(proc.StandardOutput,false));
+			var terr = new Thread(() => readerloop(proc.StandardError,false));
+
+			tout.Start();
+			terr.Start();
+
 			proc.WaitForExit();
 
 			return new RunResults