ffmpeg: set loglevel to info in debug mode
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Occasionally, the FFmpeg invocation fails (which could be due to bugs in you-get; see #1558 for instance), but `-loglevel quiet` means nothing is printed other than the exit status (pretty much always 1) in Python's traceback, which is not helpful at all.
This commit restores FFmpeg's regular output (`-loglevel info`) when `--debug` is specified. We're not using `verbose`, `debug` or `trace` because those levels are mostly only useful for debugging FFmpeg itself, which is not our goal.
Due to lack of meaningful API to access the global logging level, this is a hack based on two assumptions:
1. When `--debug` is enabled, the root logger level is set to `DEBUG`;
2. `processor.ffmpeg` is lazily imported, after command line options are parsed.
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