The FFMPEG reader fetches frames with the wrong time

[Zulko]

If I want the 300th frame and I call myreader.get_data(300) I will not get the 300th frame, but the frame from one second before. This is because this line in ffmpeg.py reads

iargs = ['-ss', "%.03f" % (starttime-offset)]

Where the offset is 1. Actually, after reaching starttime-offset, it should advance of 'offset' seconds to be exactly on 'starttime', like this:

i_arg = ['-ss', "%.06f" % (starttime - offset),
      '-i', self.filename,
      '-ss', "%.06f" % offset]

See here for details and explanations.

[almarklein]

Care for a PR?