libs: encoder: h264: support ICQ/QVBR bitrate control mode

libs: encoder: h264: support ICQ/QVBR bitrate control mode

ICQ is Intelligent Constant Quality. It will use the initial QP
vaule of icq-quality-factor to adjust QP at MB level intelligently
to improve subjective quality.

QVBR is Quality defined VBR. It will use qvbr-quality-factor to
adjust QP for each MB to get enough quality picture without waste
of bits.