Contrained encoding support

[felipewd]

Hi,

I'm a new user and trying to understand the transcoding options available for ABR outputs.

Is constrained encoding either supported or planned? -crf 23 -maxrate 6M for 1080p, for instance.*

I'm asking because we've found out this kind of encoding yields pretty good results for our videos (greater compaction for live streams but enforcing a ceiling for the ABR ladder), for both AVC and HEVC outputs.

Forgive me if this was an already discussed topic, but I couldn't anywhere in previous issues.

Great code, by the way! We're testing it thoroughly for our needs :-)

*https://trac.ffmpeg.org/wiki/Encode/H.264#ConstrainedencodingVBVmaximumbitrate