Set cabac as default entropy coder for h264.
Currently, we use CAVLC as the default entropy coder.
The CAVLC has 17% bdrate loss compare to CABAC in some typical streams. ffmpeg vaapi already uses CABAC as the default coder.
Could we change the default to CABAC? or we can add an auto mode, let vaapi encoder choose the best coder type based on encoder profile.