avformat/matroska: write FlagInterlaced element in WebM

It's listed as supported in both https://www.webmproject.org/docs/container/
and https://matroska.org/technical/specs/index.html

Reviewed-by: Dave Rice <dave@dericed.com>
Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/matroskaenc.c

@@ -791,7 +791,7 @@ static int mkv_write_video_color(AVIOContext *pb, AVCodecParameters *par, AVStre
     return 0;
 }
 
-static void mkv_write_field_order(AVIOContext *pb,
+static void mkv_write_field_order(AVIOContext *pb, int mode,
                                   enum AVFieldOrder field_order)
 {
     switch (field_order) {
@@ -809,6 +809,7 @@ static void mkv_write_field_order(AVIOContext *pb,
     case AV_FIELD_BT:
         put_ebml_uint(pb, MATROSKA_ID_VIDEOFLAGINTERLACED,
                       MATROSKA_VIDEO_INTERLACE_FLAG_INTERLACED);
+        if (mode != MODE_WEBM) {
             switch (field_order) {
             case AV_FIELD_TT:
                 put_ebml_uint(pb, MATROSKA_ID_VIDEOFIELDORDER,
@@ -829,6 +830,7 @@ static void mkv_write_field_order(AVIOContext *pb,
             }
         }
     }
+}
 
 static int mkv_write_stereo_mode(AVFormatContext *s, AVIOContext *pb,
                                  AVStream *st, int mode, int *h_width, int *h_height)
@@ -1088,8 +1090,7 @@ static int mkv_write_track(AVFormatContext *s, MatroskaMuxContext *mkv,
         put_ebml_uint (pb, MATROSKA_ID_VIDEOPIXELWIDTH , par->width);
         put_ebml_uint (pb, MATROSKA_ID_VIDEOPIXELHEIGHT, par->height);
 
-        if (mkv->mode != MODE_WEBM)
-            mkv_write_field_order(pb, par->field_order);
+        mkv_write_field_order(pb, mkv->mode, par->field_order);
 
         // check both side data and metadata for stereo information,
         // write the result to the bitstream if any is found