Read h264 headers from v4l2 to allow stream-copying.

Fixes ticket #2882.
Analyzed and tested by William C Bonner.
Merge remote-tracking branch 'cehoyos/master'

* cehoyos/master:
  Read h264 headers from v4l2 to allow stream-copying.

Merged-by: Michael Niedermayer <michaelni@gmx.at>