cslg: Add Composition Shift Least Greatest Atom

This simply add fourcc and dump function for the cslg Atom.

https://bugzilla.gnome.org/show_bug.cgi?id=751103
diff --git a/gst/isomp4/qtdemux_types.c b/gst/isomp4/qtdemux_types.c
index 6672efe..ecd02f2 100644
--- a/gst/isomp4/qtdemux_types.c
+++ b/gst/isomp4/qtdemux_types.c
@@ -146,6 +146,7 @@
   {FOURCC_rdrf, "rdrf", 0,},
   {FOURCC__gen, "Custom Genre", QT_FLAG_CONTAINER,},
   {FOURCC_ctts, "Composition time to sample", 0, qtdemux_dump_ctts},
+  {FOURCC_cslg, "Composition Shift Least Greatest", 0, qtdemux_dump_cslg},
   {FOURCC_XiTh, "XiTh", 0},
   {FOURCC_XdxT, "XdxT", 0},
   {FOURCC_loci, "loci", 0},