fix superframe index with lagged encoding

If a superframe (ARF) is generated while flushing the lagged frames
at the end of the clip, the buffer pointer wasn't being properly
updated to account for the size of the index, causing the next
frame to overwrite the index on the previous frame.

Change-Id: Ib158cc8e4183d663bdfb9ba002dd4c98916abdc9

vp9/vp9_cx_iface.c

@@ -582,7 +582,7 @@ static void pick_quickcompress_mode(vpx_codec_alg_priv_t  *ctx,
 }
 
 
-static void write_superframe_index(vpx_codec_alg_priv_t *ctx) {
+static int write_superframe_index(vpx_codec_alg_priv_t *ctx) {
   uint8_t marker = 0xc0;
   int mag, mask, index_sz;
 
@@ -619,6 +619,7 @@ static void write_superframe_index(vpx_codec_alg_priv_t *ctx) {
     *x++ = marker;
     ctx->pending_cx_data_sz += index_sz;
   }
+  return index_sz;
 }
 
 static vpx_codec_err_t vp8e_encode(vpx_codec_alg_priv_t  *ctx,
@@ -818,7 +819,7 @@ static vpx_codec_err_t vp8e_encode(vpx_codec_alg_priv_t  *ctx,
             ctx->pending_frame_sizes[ctx->pending_frame_count++] = size;
             ctx->pending_frame_magnitude |= size;
             ctx->pending_cx_data_sz += size;
-            write_superframe_index(ctx);
+            size += write_superframe_index(ctx);
             pkt.data.frame.buf = ctx->pending_cx_data;
             pkt.data.frame.sz  = ctx->pending_cx_data_sz;
             ctx->pending_cx_data = NULL;