- `-hwaccel auto` now rides along only with a hardware encoder: a
d3d11va-decoded surface cannot feed libx264 without an explicit
hwdownload/format filter chain, so the software path failed on Windows
(the only platform that ships an ffmpeg CLI) even after the encoder
probe correctly fell back;
- keep the last ffmpeg stderr lines and put them into the task error and
the console, so a failed proxy generation says why instead of a bare
'ffmpeg failed to generate proxy'.
The argument tests track the new split (software: no -hwaccel; hardware:
-hwaccel auto retained).
Mike-Solar
8f6527887d