Encode superframes in front

This moves the superframe index at the beginning of the frame packet.
There's no change in the superframe index format, and it's still
optional.

BUG=aomedia:22

Change-Id: I1488429bd114c220ed110be34baee19191ff730e
diff --git a/av1/av1_dx_iface.c b/av1/av1_dx_iface.c
index b8e05bb..98923d8 100644
--- a/av1/av1_dx_iface.c
+++ b/av1/av1_dx_iface.c
@@ -625,10 +625,14 @@
     if (res != AOM_CODEC_OK) return res;
   }
 
+  int index_size = 0;
   res = av1_parse_superframe_index(data, data_sz, frame_sizes, &frame_count,
-                                   ctx->decrypt_cb, ctx->decrypt_state);
+                                   &index_size, ctx->decrypt_cb,
+                                   ctx->decrypt_state);
   if (res != AOM_CODEC_OK) return res;
 
+  data_start += index_size;
+
   if (ctx->frame_parallel_decode) {
     // Decode in frame parallel mode. When decoding in this mode, the frame
     // passed to the decoder must be either a normal frame or a superframe with