Revert "avformat: don't mark buffer as need to be writable, as we don't write into it directly"
Revert "avformat: don't mark buffer as need to be writable, as we don't write into it directly"

This reverts commit 75890fb5e9086d91e92ca6d9abb13a307278238d.

This caused infinite loops in aviobuf when muxing to mkv.
write_flag seems to mean "mux" versus "demux".
The real meaning might be lost on us for lack of a libavformat documentation translator.

vlc-mirror/vlc,vlc-mirror/vlc,xkfz007/vlc,krichter722/vlc,xkfz007/vlc,krichter722/vlc,krichter722/vlc,krichter722/vlc,shyamalschandra/vlc,vlc-mirror/vlc,jomanmuk/vlc-2.2,vlc-mirror/vlc,vlc-mirror/vlc,jomanmuk/vlc-2.2,vlc-mirror/vlc,krichter722/vlc,jomanmuk/vlc-2.2,shyamalschandra/vlc,xkfz007/vlc,jomanmuk/vlc-2.2,xkfz007/vlc,shyamalschandra/vlc,shyamalschandra/vlc,xkfz007/vlc,vlc-mirror/vlc,shyamalschandra/vlc,xkfz007/vlc,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,xkfz007/vlc,krichter722/vlc,shyamalschandra/vlc,jomanmuk/vlc-2.2,krichter722/vlc,shyamalschandra/vlc