Spawn ffmpeg and return a Promise that resolves when the process exits successfully, or rejects with FFmpegSpawnError on failure.
Rejects with FFmpegTimeoutError when a timeout elapses and with FFmpegAbortError when opts.signal fires.
opts.signal
Spawn ffmpeg and return a Promise that resolves when the process exits successfully, or rejects with FFmpegSpawnError on failure.
Rejects with FFmpegTimeoutError when a timeout elapses and with FFmpegAbortError when
opts.signalfires.