It would be 200kbps bitrate with gop size = 12 by default
which generated too many IDR frames in rather low bit rate.
The quality would be poor.

Set these default values to -1 to check whether it's specified
by user explicitly.

Use 2Mbps bitrate as nvenc sugguested, and leave gop size
untouched in libopenh264.
Signed-off-by: Linjie Fu <linjie.fu@intel.com>
Signed-off-by: Martin Storsjö <martin@martin.st>