ffmpeg_qsv: Fix hwaccel transcoding

Set up the encoder with a hardware context which will match the one
the decoder will use when it starts later.

Includes 02c2761973dfc886e94a60a9c7d6d30c296d5b8c, with additional
hackery to get around a3a0230a9870b9018dc7415ae5872784d524cfe5 being
skipped.
