demux_raw: set keyframe flag

This is for uncompressed data, so every frame is a "keyframe". This is
part of making this demuxer work with the demuxer layer caching and
backward playback.

demux/demux_raw.c

@@ -280,6 +280,7 @@ static bool raw_read_packet(struct demuxer *demuxer, struct demux_packet **pkt)
         return true;
     }
 
+    dp->keyframe = true;
     dp->pos = stream_tell(demuxer->stream);
     dp->pts = (dp->pos  / p->frame_size) / p->frame_rate;