The special path in build_inter_predictors for chroma blocks
corresponding to sub8x8 luma blocks would always fetch the
scale factors from 'xd', which correspond (at least in the
decoder) to the references of the block for which is_chroma_reference
returns true.

The correct behaviour is to fetch the scale factors from 'ref_buf',
which corresponds to the references of the block currently being
predicted.

This patch fixes some encode/decode mismatches which were caused
by the above behaviour when the various reference frames had
different sizes.

Change-Id: I48a0a167ea25d47d08018016cf8b77885b3b5d6b