This allows the following usages:
FFMPEG_DATADIR=presets ./ffmpeg -f lavfi -i testsrc=d=5 -vcodec libx264 -vpre ipod640 -f null -
FFMPEG_DATADIR=presets ./ffmpeg -f lavfi -i testsrc=d=5 -vpre libx264-ipod640 -f null -
The second example was broken even if documented.
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