hardware: bind vaapi filter device for FFmpeg 8

[kugaevsky]

hardware: bind vaapi filter device for FFmpeg 8

Problem

#hardware=vaapi transcoding is broken on FFmpeg 8. The VAAPI branch of
MakeHardware inserts a format=vaapi|nv12,hwupload filter but relies on
-hwaccel vaapi to provide the device to the filter graph. FFmpeg 8 no
longer derives the filter device from -hwaccel alone, so hwupload fails
whenever the input isn't hardware-decoded (e.g. an RTSP restream):

[hwupload] A hardware device reference is required to upload frames to.
[AVFilterGraph] Error initializing filters
Error opening output files: Invalid argument

Note the hardware probe commands already work, because they explicitly
-init_hw_device vaapi — the transcode path just didn't.

Fix

Explicitly initialise and bind a filter device in the VAAPI branch, matching
the probes:

-init_hw_device vaapi=vaapi0 -filter_hw_device vaapi0 -hwaccel vaapi ...

Applied to both VAAPI sub-branches (with and without drawtext).

Testing

Reproduced and A/B-tested on an Intel N150 (iHD driver), FFmpeg 8.0.1, same
input stream (ffmpeg:virtual?video#video=h264#hardware=vaapi):

build	result
master	[hwupload] A hardware device reference is required → stream fails
this PR	h264 1920x1080 — encodes fine

Also verified end-to-end transcoding a live H265 doorbell stream to H264 for a
browser that can't decode HEVC.

Question

-init_hw_device vaapi=vaapi0 auto-selects the DRM node (same as the probe
commands). If you'd rather thread through an explicit device path, happy to
adjust.