Upsampled references currently increase the size of references by
64 times. This patch limits the memory used by the encoder to
about 3GB when encoding high bit depth content.

This should be re-evaluated in the future, if doing 8-tap
resampling in the motion search becomes reasonably fast, or if
the upsampled references are reduced in size (by omitting some
subpel positions and interpolating them instead).

Change-Id: I6d84ff0d6202ec46f4fa53e268e68aa808e5df85