Update libx264.c to use new libx264 features

With b_keyframe instead of IDR for detecting keyframes, ffmpeg should now
support periodic encoding with periodic intra refresh (although there is no
interface option for it yet).
Set the new timebase values for full VFR input support.
Bump configure to check for API version 83.

Originally committed as revision 21317 to svn://svn.ffmpeg.org/ffmpeg/trunk