Support for transparencies in animated gifs requires modifying both

libavcodec/gif.c and libavformat/gif.c because both the graphics
control extension (handled by libavformat/gif.c) and the raw frame data
(handled by libavcodec/gif.c) must be changed. This is because
transparencies in GIF can be used both to create a transparent image,
and to provide optimization.

How transparencies are interpreted in a given frame is controlled by
the “disposal method”, which must be set appropriately in the graphics
control extension.

 The “in place” disposal method is used when transparency indicates
optimization, and the “background” disposal method is used when
transparency is intended to be preserved. In order to support both
disposal methods, libavcodec/gif.c must signal to libavformat/gif.c
which disposal method is required for every frame. This is done with a
new side data type: AV_PKT_DATA_GIF_FRAME_DISPOSAL. This requires a
change to avcodec.h

Unfortunately, the addition of a new side data type causes some of the
FATE tests to fail, so the fate tests are updated here as well.

Added additional fix for GIF transparency artifacts

libavcodec/gif.c

@@ -191,6 +191,7 @@ static void gif_crop_translucent(AVCodecContext *avctx,
 
         *height = y_end + 1 - *y_start;
         *width  = x_end + 1 - *x_start;
+
         av_log(avctx, AV_LOG_DEBUG,"%dx%d image at pos (%d;%d) [area:%dx%d]\n",
                *width, *height, *x_start, *y_start, avctx->width, avctx->height);
     }
@@ -267,7 +268,7 @@ static int gif_image_write_image(AVCodecContext *avctx,
     int bcid = -1, honor_transparency = (s->flags & GF_TRANSDIFF) && s->last_frame && !palette;
     const uint8_t *ptr;
 
-    if (!s->image && avctx->frame_number && is_image_translucent(avctx, buf, linesize)) {
+    if (!s->image && is_image_translucent(avctx, buf, linesize)) {
         gif_crop_translucent(avctx, buf, linesize, &width, &height, &x_start, &y_start);
         honor_transparency = 0;
         disposal = GCE_DISPOSAL_BACKGROUND;
@@ -497,4 +498,4 @@ AVCodec ff_gif_encoder = {
         AV_PIX_FMT_GRAY8, AV_PIX_FMT_PAL8, AV_PIX_FMT_NONE
     },
     .priv_class     = &gif_class,
-};
+};