Add VAAPI Encoder Support

[e-dong]

Useful for those who encode on VAAPI rather than AMD AMF

Reference: https://trac.ffmpeg.org/wiki/Hardware/VAAPI

[Proryanator]

I don't have access to an AMD GPU anymore to be able to fully test this feature out, however I could always code it in with a note about it being experimental.

[geerlingguy]

Subscribing — today I was working on testing a couple AMD GPUs, and had to resort to running ffmpeg directly since the amf encoder is only supported with the 'Pro' driver (which I can't install on my particular setup).

Here's the ffmpeg command I ran for the 720p60 encode:

ffmpeg -init_hw_device vaapi=foo:/dev/dri/renderD129 -i 720-60.y4m -filter_hw_device foo -vf 'format=nv12,hwupload' -c:v h264_vaapi -pix_fmt yuv420p -movflags +faststart 720-60.mp4

On my ffmpeg install I see the encoders:

 V....D h264_nvenc           NVIDIA NVENC H.264 encoder (codec h264)
 V..... h264_v4l2m2m         V4L2 mem2mem H.264 encoder wrapper (codec h264)
 V....D h264_vaapi           H.264/AVC (VAAPI) (codec h264)
 V....D h264_vulkan          H.264/AVC (Vulkan) (codec h264)

This is on a Raspberry Pi; I don't expect every encoder under the sun to be supported of course, but I think vaapi might be more widespread than h264_amf/hevc_amf at least.

(I'd be willing to test, too, if it lines up with some of my testing cycles.)

[mardaravicius]

attempt to add support for it:
#82