qtmux: move subtitle layer above video and set alternate group
layer -1 is above video, that is 0
And having all subtitles in alternate group 2 means that only one
should be selected at a time.
https://bugzilla.gnome.org/show_bug.cgi?id=737095
diff --git a/gst/isomp4/atoms.c b/gst/isomp4/atoms.c
index f8bf050..b27abc7 100644
--- a/gst/isomp4/atoms.c
+++ b/gst/isomp4/atoms.c
@@ -3426,6 +3426,9 @@
{
atom_trak_set_subtitle (trak, context);
trak->mdia.mdhd.time_info.timescale = 1000;
+
+ trak->tkhd.alternate_group = 2; /* same for all subtitles */
+ trak->tkhd.layer = -1; /* above video (layer 0) */
}
void
