fix last frame buffer copy logic regression

Commit 0ce39012 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 0ce39012 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