[NORMATIVE, fwd-kf] Signal refresh_frame_flags for fwd keyframes

Unlike shown keyframes, forward keyframes do not want to overwrite
all of the reference slots immediately on decoding. This is so that
we can use both the "previous" and "next" keyframes as references
while decoding intermediate frames.

To support this, we need to make two changes:

* Non-shown keyframes should explicitly signal which reference
  slots to overwrite;

* We shouldn't overwrite all of the frame context slots at the
  start of decoding a forward key frame (in av1_setup_frame_contexts()).
  Instead, we should only overwrite the special 'default' context,
  just as for intra-only or error-resilient frames.

  In fact, it seems that we don't need to overwrite all the frame
  contexts even for shown key frames, since we will update all of
  the reference slots to point to the correct frame context once
  we're finished decoding the frame anyway.

BUG=aomedia:1509

Change-Id: I3a4e90a0b9dceab25f94312bd4ecc96ea016f624

av1/common/entropymode.c

diff --git a/av1/common/entropymode.c b/av1/common/entropymode.c
index a2a542c..ae8dab0 100644
--- a/av1/common/entropymode.c
+++ b/av1/common/entropymode.c
@@ -1186,21 +1186,16 @@
 }
 
 void av1_setup_frame_contexts(AV1_COMMON *cm) {
-  int i;
 #if CONFIG_NO_FRAME_CONTEXT_SIGNALING
-  if (cm->frame_type == KEY_FRAME) {
-    // Reset all frame contexts, as all reference frames will be lost.
-    for (i = 0; i < FRAME_CONTEXTS; ++i) cm->frame_contexts[i] = *cm->fc;
-  } else if (frame_is_intra_only(cm) || cm->error_resilient_mode) {
-    // Store the frame context into a special slot (not associated with any
-    // reference buffer), so that we can set up cm->pre_fc correctly later
-    cm->frame_contexts[FRAME_CONTEXT_DEFAULTS] = *cm->fc;
-  }
+  assert(frame_is_intra_only(cm) || cm->error_resilient_mode);
+  // Store the frame context into a special slot (not associated with any
+  // reference buffer), so that we can set up cm->pre_fc correctly later
+  cm->frame_contexts[FRAME_CONTEXT_DEFAULTS] = *cm->fc;
 #else
   if (cm->frame_type == KEY_FRAME || cm->error_resilient_mode ||
       cm->reset_frame_context == RESET_FRAME_CONTEXT_ALL) {
     // Reset all frame contexts.
-    for (i = 0; i < FRAME_CONTEXTS; ++i) cm->frame_contexts[i] = *cm->fc;
+    for (int i = 0; i < FRAME_CONTEXTS; ++i) cm->frame_contexts[i] = *cm->fc;
   } else if (cm->reset_frame_context == RESET_FRAME_CONTEXT_CURRENT) {
     // Reset only the frame context specified in the frame header.
     cm->frame_contexts[cm->frame_context_idx] = *cm->fc;

av1/decoder/decodeframe.c

diff --git a/av1/decoder/decodeframe.c b/av1/decoder/decodeframe.c
index efbffec..0071fc5 100644
--- a/av1/decoder/decodeframe.c
+++ b/av1/decoder/decodeframe.c
@@ -2778,7 +2778,12 @@
 
   if (cm->frame_type == KEY_FRAME) {
     wrap_around_current_video_frame(pbi);
-    pbi->refresh_frame_flags = (1 << REF_FRAMES) - 1;
+#if CONFIG_FWD_KF
+    if (!cm->show_frame)  // unshown keyframe (forward keyframe)
+      pbi->refresh_frame_flags = aom_rb_read_literal(rb, REF_FRAMES);
+    else  // shown keyframe
+#endif    // CONFIG_FWD_KF
+      pbi->refresh_frame_flags = (1 << REF_FRAMES) - 1;
 
     for (int i = 0; i < INTER_REFS_PER_FRAME; ++i) {
       cm->frame_refs[i].idx = INVALID_IDX;