demux, vd_ffmpeg: fix demux keyframe flag, set AV_PKT_FLAG_KEY

demux, vd_ffmpeg: fix demux keyframe flag, set AV_PKT_FLAG_KEY

There was some confusion about the "flags" field in demuxer packets.
Demuxers set it to either 1 or 0x10 to indicate a keyframe (and the
field was not used to indicate anything else). This didn't cause
visible problems because nothing read the value. Replace the "flags"
field with a boolean "keyframe" field. Set AV_PKT_FLAG_KEY based on
this field in packets fed to libavcodec video decoders (looks like PNG
and ZeroCodec are the only ones which depend on values from demuxer;
previously this was hardcoded to true for PNG).

Make demux_mf set the keyframe field in every packet. This matters for
PNG files now that the demuxer flag is forwarded to libavcodec.

Fix logic setting the field in demux_mkv. It had probably not been
updated when adding SimpleBlock support. This probably makes no
difference for any current practical use.