Radeonsi driver does not support packed headers

Nobody has claimed this yet.

This leads to us being unable to mux into containers that expect a global header such as Matroska. Our transcoding example outputs Matroska, so we cannot use AMD cards with the radeonsi driver with this particular example, eg:

```
go run cmd/transcoding/transcoding.go transcoder/test.ts P720p30fps16x9,P576p30fps16x9,P360p30fps16x9,P240p30fps16x9 va /dev/dri/renderD129
```

Note that we encounter the exact same problem with ffmpeg, since it's a shortcoming of the driver:

```
ffmpeg -hwaccel vaapi -hwaccel_output_format vaapi -hwaccel_device /dev/dri/renderD129 -i ~/josh/lpms/transcoder/test.ts -c:v h264_vaapi out/output.mkv
```

Fixing this involves making improvements to the Mesa radeon driver. Would be great to have this in one day.

QuickSync using the Intel media-driver (iHD) works fine (/dev/dri/renderD128 on the machine in the NY office).

Start with cmd/transcoding/transcoding.go and reproduce the Matroska failure using transcoder/test.ts and the radeonsi device shown in the issue. The reported fix requires improvements to the Mesa radeon driver; done means packed headers work for the transcoding example, with Matroska output succeeding on AMD radeonsi.

Written by the indexing model from the issue text.