avformat/ivfenc: fix writing codec tag

The value in AVCodecParameters->codec_tag may not be correct for IVF,
as it's the case when remuxing AV1 streams from mp4, so ignore it and
write the correct value based on codec ID instead.
Reviewed-by: Jan Ekström <jeebjp@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/ivfenc.c

@@ -46,7 +46,7 @@ static int ivf_write_header(AVFormatContext *s)
     avio_write(pb, "DKIF", 4);
     avio_wl16(pb, 0); // version
     avio_wl16(pb, 32); // header length
-    avio_wl32(pb, par->codec_tag ? par->codec_tag :
+    avio_wl32(pb,
               par->codec_id == AV_CODEC_ID_VP9 ? AV_RL32("VP90") :
               par->codec_id == AV_CODEC_ID_VP8 ? AV_RL32("VP80") : AV_RL32("AV01"));
     avio_wl16(pb, par->width);