av1_vaapi does not respect qp, despite implementing qp as a viable rc_mode

[tjhexf]

Summary of the bug:
How to reproduce:

ffmpeg -i input.mkv -vaapi_device /dev/dri/renderD128 -vf format='nv12,hwupload' -c:v av1_vaapi -qp:v 40 output.mkv

ffmpeg will fail to use qp, writing:

[out#0/matroska @ 0x3c25140] Codec AVOption qp (Constant QP (for P-frames; scaled by qfactor/qoffset for I/B)) has not been used for any stream. The most likely reason is either wrong type (e.g. a video option with no video streams) or that it is a private option of some encoder which was not actually used for any stream.

Logs imply it's using qp 25, despite not allowing for overriding this.

[Fei Wang]

Please use "-global_quality xxx". Same with vp8/vp9_vaapi.

[tjhexf]

Replying to wangfei:

Please use "-global_quality xxx". Same with vp8/vp9_vaapi.

Thanks, i wonder why it isn't in the help command for av1_vaapi

[Fei Wang]

Replying to tjhexf:

Replying to wangfei:

Please use "-global_quality xxx". Same with vp8/vp9_vaapi.

Thanks, i wonder why it isn't in the help command for av1_vaapi

It's a common option for AVCodecContext, should not be duplicate in private encoder.

[Fei Wang]

Some more details on options available for VAAPI encoders:
​http://www.ffmpeg.org/ffmpeg-codecs.html#VAAPI-encoders