Force automatic thread_count to 1 for cbr mjpeg frame threading.

Constant bit rate mjpeg encoding fails for threads > 1 and frame threading.
Merge remote-tracking branch 'cehoyos/master'

* cehoyos/master:
  Force automatic thread_count to 1 for cbr mjpeg frame threading.

Merged-by: Michael Niedermayer <michaelni@gmx.at>
Warn the user if mjpeg cbr encoding with frame threading was requested.