fix last frame buffer copy logic regression

Commit 0ce3901 introduced a change in the frame buffer copy logic where
the NEW frame could be copied to the ARF or GF buffer through the
copy_buffer_to_{arf,gf}==1 flags, if the LAST frame was not being
refreshed. This is not correct. The intent of the
copy_buffer_to_{arf,gf}==1 flag is to copy the LAST buffer. To copy the
NEW buffer, the refresh_{alt_ref,golden}_frame flag should be used.

The original buffer copy logic is fairly convoluted. For example:

    if (cm->refresh_last_frame)
    {
        vp8_swap_yv12_buffer(&cm->last_frame, &cm->new_frame);

        cm->frame_to_show = &cm->last_frame;
    }
    else
    {
        cm->frame_to_show = &cm->new_frame;
    }
    ...
    if (cm->copy_buffer_to_arf)
    {
        if (cm->copy_buffer_to_arf == 1)
        {
            if (cm->refresh_last_frame)
                vp8_yv12_copy_frame_ptr(&cm->new_frame, &cm->alt_ref_frame);
            else
                vp8_yv12_copy_frame_ptr(&cm->last_frame, &cm->alt_ref_frame);
        }
        else if (cm->copy_buffer_to_arf == 2)
            vp8_yv12_copy_frame_ptr(&cm->golden_frame, &cm->alt_ref_frame);
    }

Effectively, if refresh_last_frame, then new and last are swapped, so
when "new" is copied to ARF, it's equivalent to copying LAST to ARF. If
not refresh_last_frame, then LAST is copied to ARF. So LAST is copied to
ARF in both cases.

Commit 0ce3901 removed the first buffer swap but kept the
refresh_last_frame?new:last behavior, changing the sense since the first
swap wasn't done to the more readable refresh_last_frame?last:new, but
this logic is not correct when !refresh_last_frame.

This commit restores the correct behavior from v0.9.1 and prior. This
case is missing from the test vector set.

Change-Id: I8369fc13a37ae882e31a8a104da808a08bc8428f

vp8/decoder/onyxd_if.c

@@ -254,12 +254,7 @@ static void ref_cnt_fb (int *buf, int *idx, int new_idx)
 /* If any buffer copy / swapping is signalled it should be done here. */
 static int swap_frame_buffers (VP8_COMMON *cm)
 {
-    int fb_to_update_with, err = 0;
-
-    if (cm->refresh_last_frame)
-        fb_to_update_with = cm->lst_fb_idx;
-    else
-        fb_to_update_with = cm->new_fb_idx;
+    int err = 0;
 
     /* The alternate reference frame or golden frame can be updated
      *  using the new, last, or golden/alt ref frame.  If it
@@ -271,7 +266,7 @@ static int swap_frame_buffers (VP8_COMMON *cm)
         int new_fb = 0;
 
         if (cm->copy_buffer_to_arf == 1)
-            new_fb = fb_to_update_with;
+            new_fb = cm->lst_fb_idx;
         else if (cm->copy_buffer_to_arf == 2)
             new_fb = cm->gld_fb_idx;
         else
@@ -285,7 +280,7 @@ static int swap_frame_buffers (VP8_COMMON *cm)
         int new_fb = 0;
 
         if (cm->copy_buffer_to_gf == 1)
-            new_fb = fb_to_update_with;
+            new_fb = cm->lst_fb_idx;
         else if (cm->copy_buffer_to_gf == 2)
             new_fb = cm->alt_fb_idx;
         else