[NORMATIVE, FILM_GRAIN_SHOWEX] Enable FILM_GRAIN_SHOWEX

Enable FILM_GRAIN_SHOWEX by default as it was provisionally
adopted at the WG meeting on 2018-Feb-23

Note: Enabling will add the "showable_frame" flag even if
FILM_GRAIN is disabled. Once FILM_GRAIN is also enabled
then the new FILM_GRAIN_SHOWEX film grain paramater
signalling will be used.

Change-Id: I77842ae70f1906969cfec0580383d5fa492333b1

av1/encoder/bitstream.c

diff --git a/av1/encoder/bitstream.c b/av1/encoder/bitstream.c
index 857e000..7aea432 100644
--- a/av1/encoder/bitstream.c
+++ b/av1/encoder/bitstream.c
@@ -2726,7 +2726,7 @@
     for (ref_frame = LAST_FRAME; ref_frame < TOTAL_REFS_PER_FRAME;
          ref_frame++) {
       ref_idx = get_ref_frame_map_idx(cpi, ref_frame);
-      assert(idx != INVALID_IDX);
+      assert(ref_idx != INVALID_IDX);
       buf_idx = cm->ref_frame_map[ref_idx];
       if (frame_bufs[buf_idx].film_grain_params_present &&
           memcmp(pars, &frame_bufs[buf_idx].film_grain_params, sizeof(*pars))) {