ffmpeg padding doesn't work, use -vf pad instead

Newer Versions of ffmpeg won't use padleft, padright and so on anymore. Using the padding will cause this message:

Option 'padleft' has been removed, use the pad filter instead
ffmpeg: failed to set value '0' for option 'padleft'

So the newer versions of ffmpeg uses this instead: -vf pad=0:0:0:0:black

see: #1119550: ffmpeg padding doesn't work, use -vf pad instead

This is a good moment to remove ffmpeg_converter_get_padded_size() and use only ffmpeg_wrapper_padded_size(), to only fix the problem in one place.