Hardware acceleration options

[justdanpo]

Hardware acceleration allows to dramatically reduce encoding time. Please add an option to choose encoder or autodetection of working one (not all encoders ffmpeg reports of actually work on a target machine).
List of available encoders may be got from ffmpeg (depends on ffmpeg build):

$ ffmpeg -hide_banner -encoders | grep '(codec h264)'
 V..... libx264              libx264 H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10 (codec h264)
 V..... libx264rgb           libx264 H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10 RGB (codec h264)
 V..... h264_nvenc           NVIDIA NVENC H.264 encoder (codec h264)
 V..... h264_omx             OpenMAX IL H.264 video encoder (codec h264)
 V..... h264_v4l2m2m         V4L2 mem2mem H.264 encoder wrapper (codec h264)
 V..... h264_vaapi           H.264/AVC (VAAPI) (codec h264)
 V..... nvenc                NVIDIA NVENC H.264 encoder (codec h264)
 V..... nvenc_h264           NVIDIA NVENC H.264 encoder (codec h264)

[justdanpo]

Here is how certain encoder may be tested:
ffmpeg -f lavfi -i nullsrc=s=320x128:d=0 -c:v h264_amf -f null -