Ensure all ffmpeg instances are shut down in application close / crash

[nikeee]

We don't want anything left. Especially when it records stuff.

[nikeee]

Well, we kill the process in IDisposable.Dispose now (wasn't possible before with FFmpegCore). That doesn't suffice, but is better than nothing, I guess.

There is a more complex way of killing child processes on application crash that involves some Win32 APIs, called "Job Objects": https://stackoverflow.com/questions/3342941

This won't block the 2.0 release for now, as we can do this later.