This reverts commit a194876.

- There is no longer a singular FFmpeg process, because outside of the conversion function, it was only used during EngineBase.Dispose, where nothing happened because the conversion function disposes it
- Added thread safety to EnsureFFmpegFileExists
- Changed default path of the ffmpeg executable to be in the temporary directory, to avoid littering the root of the drive.  Included in this path is a new Guid, which will allow it to be cleaned up without affecting other instances of Engine
- Removed the event handler from FFmpegProcess.ErroDataReceived.  In general (I don't know about this particular case), not removing event handlers can prevent the garbage collector from cleaning up the class