qtdemux: extract cover art images into GST_TAG_IMAGE not PREVIEW_IMAGE

These are usually much bigger than icon size and required by
iTunes to be certain fairly large sizes. In qtmux it is also
the IMAGE tags which we write out as 'covr' atoms.

gst/isomp4/qtdemux.c

@@ -12429,7 +12429,7 @@ static const struct
   FOURCC__gen, GST_TAG_GENRE, NULL, qtdemux_tag_add_str}, {
   FOURCC_gnre, GST_TAG_GENRE, NULL, qtdemux_tag_add_gnre}, {
   FOURCC_tmpo, GST_TAG_BEATS_PER_MINUTE, NULL, qtdemux_tag_add_tmpo}, {
-  FOURCC_covr, GST_TAG_PREVIEW_IMAGE, NULL, qtdemux_tag_add_covr}, {
+  FOURCC_covr, GST_TAG_IMAGE, NULL, qtdemux_tag_add_covr}, {
   FOURCC_sonm, GST_TAG_TITLE_SORTNAME, NULL, qtdemux_tag_add_str}, {
   FOURCC_soal, GST_TAG_ALBUM_SORTNAME, NULL, qtdemux_tag_add_str}, {
   FOURCC_soar, GST_TAG_ARTIST_SORTNAME, NULL, qtdemux_tag_add_str}, {