avformat/img2: Add support for AVIF mux in image2

Add support for AVIF muxing in the image2 muxer.

Tested with this example:
ffmpeg -lavfi testsrc=duration=1:size=320x320 -g 1 -flags global_header -c:v libaom-av1 -f image2 img-%2d.avif

Signed-off-by: Vignesh Venkatasubramanian <vigneshv@google.com>

libavformat/img2enc.c

@@ -58,6 +58,8 @@ static int write_header(AVFormatContext *s)
         img->muxer = "gif";
     } else if (st->codecpar->codec_id == AV_CODEC_ID_FITS) {
         img->muxer = "fits";
+    } else if (st->codecpar->codec_id == AV_CODEC_ID_AV1) {
+        img->muxer = "avif";
     } else if (st->codecpar->codec_id == AV_CODEC_ID_RAWVIDEO) {
         const char *str = strrchr(s->url, '.');
         img->split_planes =     str
@@ -265,7 +267,7 @@ const AVOutputFormat ff_image2_muxer = {
     .long_name      = NULL_IF_CONFIG_SMALL("image2 sequence"),
     .extensions     = "bmp,dpx,exr,jls,jpeg,jpg,jxl,ljpg,pam,pbm,pcx,pfm,pgm,pgmyuv,"
                       "png,ppm,sgi,tga,tif,tiff,jp2,j2c,j2k,xwd,sun,ras,rs,im1,im8,"
-                      "im24,sunras,vbn,xbm,xface,pix,y",
+                      "im24,sunras,vbn,xbm,xface,pix,y,avif",
     .priv_data_size = sizeof(VideoMuxData),
     .video_codec    = AV_CODEC_ID_MJPEG,
     .write_header   = write_header,