Async depth will allow qsv filter cache few frames, and avoid force
switch and end filter task frame by frame. This change will improve
performance for some multi-task case, for example 1:N transcode(
decode + vpp + encode) with all QSV plugins.

Performance data test on my Coffee Lake Desktop(i7-8700K) by using
the following 1:8 transcode test case improvement:
1. Fps improved from 55 to 130.
2. Render/Video usage improved from ~61%/~38% to ~100%/~70%.(Data get
from intel_gpu_top)

test CMD:
ffmpeg -v verbose -init_hw_device qsv=hw:/dev/dri/renderD128 -filter_hw_device                 \
 hw -hwaccel qsv -hwaccel_output_format qsv -c:v h264_qsv -i 1920x1080.264                     \
-vf 'vpp_qsv=w=1280:h=720:async_depth=4' -c:v h264_qsv -r:v 30 -preset 7 -g 33 -refs 2 -bf 3 -q 24 -f null - \
-vf 'vpp_qsv=w=1280:h=720:async_depth=4' -c:v h264_qsv -r:v 30 -preset 7 -g 33 -refs 2 -bf 3 -q 24 -f null - \
-vf 'vpp_qsv=w=1280:h=720:async_depth=4' -c:v h264_qsv -r:v 30 -preset 7 -g 33 -refs 2 -bf 3 -q 24 -f null - \
-vf 'vpp_qsv=w=1280:h=720:async_depth=4' -c:v h264_qsv -r:v 30 -preset 7 -g 33 -refs 2 -bf 3 -q 24 -f null - \
-vf 'vpp_qsv=w=1280:h=720:async_depth=4' -c:v h264_qsv -r:v 30 -preset 7 -g 33 -refs 2 -bf 3 -q 24 -f null - \
-vf 'vpp_qsv=w=1280:h=720:async_depth=4' -c:v h264_qsv -r:v 30 -preset 7 -g 33 -refs 2 -bf 3 -q 24 -f null - \
-vf 'vpp_qsv=w=1280:h=720:async_depth=4' -c:v h264_qsv -r:v 30 -preset 7 -g 33 -refs 2 -bf 3 -q 24 -f null -
Signed-off-by: Fei Wang <fei.w.wang@intel.com>
Reviewed-by: Linjie Fu <linjie.justin.fu@gmail.com>
Signed-off-by: Zhong Li <zhongli_dev@126.com>