When no `downloader` is passed to `FFmpegPostProcessor`
an exception was raised trying to get the prefer ffmpeg param.
AttributeError: 'NoneType' object has no attribute 'params'
This fixes and defaults to `False`.
As the "LG Time Machine" (a (not so) smart TV) has a limitation for video dimensions (as for codecs), I take to implement an extra parameter `--pp-params` where we can send extra parameterization for the video converter (post-processor).
Example:
```
$ youtube-dl --recode-video=xvid --pp-params='-s 720x480' -c https://www.youtube.com/watch?v=BE7Qoe2ZiXE
```
That works fine on a 4yo LG Time Machine.
Closes#5733
Without the '--keep-video' option the two files would be downloaded again and even using the option, ffmpeg would be run again, which for some videos can take a long time.
We use a temporary file with ffmpeg so that the final file only exists if it success