Multi-arf: Change ref buffer for primary arf.

For the primary arf in a group, if multiple arfs
are enabled and we were using arfs in the previous
group, then allow the second arf from the previous
group to be used as an additional reference.

Change-Id: Iaf41706a52f54ef21548026851cd77100d6aebda

vp9/encoder/vp9_firstpass.c

diff --git a/vp9/encoder/vp9_firstpass.c b/vp9/encoder/vp9_firstpass.c
index d0dd182..971b159 100644
--- a/vp9/encoder/vp9_firstpass.c
+++ b/vp9/encoder/vp9_firstpass.c
@@ -1360,7 +1360,8 @@
     twopass->gf_group.arf_src_offset[frame_index] =
       (unsigned char)(rc->baseline_gf_interval - 1);
     twopass->gf_group.arf_update_idx[frame_index] = arf_buffer_indices[0];
-    twopass->gf_group.arf_ref_idx[frame_index] = arf_buffer_indices[0];
+    twopass->gf_group.arf_ref_idx[frame_index] =
+      arf_buffer_indices[cpi->multi_arf_enabled && rc->source_alt_ref_active];
     ++frame_index;
 
     if (cpi->multi_arf_enabled) {