configure: add encoding to --enable-libopus doc.
avcodec/hevc: fix outputted AVFrame.key_frame

previously it was always 1

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/http: remove never twice executable loop

Fixes CID1197069

Reviewed-by: Tomas Härdin <tomas.hardin@codemill.se>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>