Implement the fields rc_max_available_vbv_use and

rc_min_vbv_overflow_use in AVCodecContext, and use their values in the
ratecontrol code rather than hardcoded ones.

See the thread: "[RFC] ratecontrol buffer size magic".
Patch by Baptiste Coudurier.

Originally committed as revision 15955 to svn://svn.ffmpeg.org/ffmpeg/trunk