First (black) frame chosen as thumbnail for some files

[sammoth]

Hi, I have some mpeg TS files where I get a black thumbnail, because ffmpegthumbnailer is choosing the first frame of the file instead of seeking to 20% or such. I get the output "stream 0 : no PTS found at end of file, duration not set". I think this might be because AVStream::start_time isn't being taken account of, as these files are chopped up from blu-ray m2ts streams and the resulting files' timestamps won't start at zero. In my own code I was able to generate thumbnails accurately for these files by basing seeks off of AVStream::start_time. I can provide an example if needed.

[dirkvdb]

I'm happy to accept a pull request. But unfortunately I have no time at the moment to perform investigation into issues.