Honour $BIZHAWK_DATA_HOME when downloading FFmpeg

also stop looking for `/ffmpeg.exe` in DiscoHawk when it's always downloaded to
`/dll/ffmpeg.exe`

Author: YoshiRulz

src/BizHawk.Client.DiscoHawk/Program.cs

@@ -5,6 +5,7 @@
 using System.IO;
 
 using BizHawk.Common;
+using BizHawk.Common.PathExtensions;
 using BizHawk.Emulation.DiscSystem;
 
 using OSTC = EXE_PROJECT.OSTailoredCode;
@@ -73,10 +74,7 @@ private static void SubMain(string[] args)
 				OSTC.LinkedLibManager.FreeByPtr(lib);
 			}
 
-			var ffmpegPath = Path.Combine(GetExeDirectoryAbsolute(), "ffmpeg.exe");
-			if (!File.Exists(ffmpegPath))
-				ffmpegPath = Path.Combine(Path.Combine(GetExeDirectoryAbsolute(), "dll"), "ffmpeg.exe");
-			FFmpegService.FFmpegPath = ffmpegPath;
+			FFmpegService.FFmpegPath = Path.Combine(PathUtils.DataDirectoryPath, "dll", OSTC.IsUnixHost ? "ffmpeg" : "ffmpeg.exe");
 
 			if (args.Length == 0)
 			{

src/BizHawk.Client.EmuHawk/AVOut/FFmpegDownloaderForm.cs

@@ -89,6 +89,8 @@ private void Download()
 						if (exiting)
 							return;
 
+						DirectoryInfo parentDir = new(Path.GetDirectoryName(FFmpegService.FFmpegPath)!);
+						if (!parentDir.Exists) parentDir.Create();
 						File.WriteAllBytes(FFmpegService.FFmpegPath, data);
 						if (OSTailoredCode.IsUnixHost)
 						{

src/BizHawk.Client.EmuHawk/Program.cs

@@ -155,7 +155,7 @@ private static int SubMain(string[] args)
 			initialConfig.ResolveDefaults();
 			// initialConfig should really be globalConfig as it's mutable
 
-			FFmpegService.FFmpegPath = Path.Combine(PathUtils.DllDirectoryPath, OSTC.IsUnixHost ? "ffmpeg" : "ffmpeg.exe");
+			FFmpegService.FFmpegPath = Path.Combine(PathUtils.DataDirectoryPath, "dll", OSTC.IsUnixHost ? "ffmpeg" : "ffmpeg.exe");
 
 			StringLogUtil.DefaultToDisk = initialConfig.Movies.MoviesOnDisk;