The vp8_post_proc_down_and_across_mb_row_sse2() needs space for an
even number of macroblocks, as they are read two at a time for the
chroma planes. Round up the width during the allocation of
pp_limits_buffer to support this.

Change-Id: Ibfc10c7be290d961ab23ac3dde12a7bb96c12af0