Transcode support

Prompted by the suggestion of Phil Grundig I might be adding transcode as a capturer in the future.

He also provided examples:

Frame extraction:

  $ transcode -o thumb  -y jpg,null -i somevideo.mpg -c 0-1,299-300

Will take frames 1 and 300 as jpegs

Frame extraction with size reduction:

  $ transcode -r 5 -o thumb  -y jpg,null -i somevideo.mpg -c 0-1,299-300

-r reduces size, e.g. -r 5 reduces size one-fifth.