Coding path sync: filter packets from encoder.

We're only interested in AOM_CODEC_CX_FRAME_PKT, but the encoder might
output packets from other types (e.g when CONFIG_INTERNAL_STATS).

Change-Id: I1c153ca7519067dc82cb76b131b09a537bb37f62

test/coding_path_sync.cc

@@ -64,7 +64,14 @@ class CompressedSource {
     aom_codec_encode(&enc_, &img, frame_count_++, 1, 0, 0);
 
     aom_codec_iter_t iter = NULL;
-    return aom_codec_get_cx_data(&enc_, &iter);
+
+    const aom_codec_cx_pkt_t *pkt = NULL;
+
+    do {
+      pkt = aom_codec_get_cx_data(&enc_, &iter);
+    } while (pkt && pkt->kind != AOM_CODEC_CX_FRAME_PKT);
+
+    return pkt;
   }
 
  private: