Without this patch, the experiment of ext-refs showed almost no coding
gains compared to the baseline. This is because when ext-refs is on, the
use of upsampled reference is off.

With this patch, the ext-refs experiment works with the upsampled
references and shows coding gains in Overall PSNR as follows, with ~5%
slow down for encoding time:

lowres: Avg - -0.965;  BDRate - -0.844
derflr: Avg - -0.847;  BDRate - -0.669

Note that the previous patch a912c6ec
that "Make LAST_FRAME always point to the newly coded frame in ext-refs"
made ext-refs work with the upsampled refereces.

Change-Id: Id79248d71760109fb9198af4f45718b17455555f