[Feature]: Retain HDR+10 metadata

### What Feature?

```bash
$ ffmpeg -hwaccel vaapi -hwaccel_output_format vaapi -init_hw_device vaapi:/dev/dri/renderD129,driver=iHD -i before.hevc after.hevc &> /dev/null
$ mediainfo before.hevc | grep HDR
HDR format : SMPTE ST 2094 App 4, Version 1, HDR10+ Profile B compatible
$ mediainfo after.hevc | grep HDR
HDR format : SMPTE ST 2086, HDR10 compatible
```

### What's the usage scenario would be benifited?

Transcode for media delivery

### What impacted?

Everytime Plex transcodes media you'll lose the HDR10+ metadata. It'd be nice if VA-API was able to retain the metadata but if that's not possible I'd also be willing to provide the info myself (`metadata.json`).

### Do you want to contribute a patch to develop this feature?

No.