feature request: bitrate modes

[cedspam]

ffmpeg have options for abr,vbr and constant quantizers for bitrate control

[utack]

I think compared to the current "bitrate" field two other modes are interesting for mobile application:

• target size, for when someone needs to send a file to a service with size limit or targets to limit their mobile data use: it should calculate the bitrate and do a 2pass with turbo firstpass
• constant quality: for when the video should look nice, the encoder should then use a constant quality option and let the bitrate automatically decide

[licaon-kter]

56 ?