plug-ins: fix #8553 GIMP 2.99.12 Cannot Open WebP Files

GIMP tried to open webp files as ani (animated cursor) files. The reason
for this is that for ani we had set the file magic as the first 4 bytes
should be RIFF.

However, RIFF is a container format, used by many different file formats,
among which is also webp.
This means that checking for RIFF is not specific enough.

Based on the info on http://fileformats.archiveteam.org/wiki/ANI
we will check the 4 bytes starting at offset 8 for ACON, which is
apparently the identifying part for animated icons.

plug-ins/file-ico/ico.c

@@ -242,7 +242,7 @@ ico_create_procedure (GimpPlugIn  *plug_in,
       gimp_file_procedure_set_extensions (GIMP_FILE_PROCEDURE (procedure),
                                           "ani");
       gimp_file_procedure_set_magics (GIMP_FILE_PROCEDURE (procedure),
-                                      "0,string,RIFF");
+                                      "8,string,ACON");
 
       gimp_load_procedure_set_thumbnail_loader (GIMP_LOAD_PROCEDURE (procedure),
                                                 LOAD_ANI_THUMB_PROC);