Amends previous commit to also handle subsampling correctly.
Change ID of prev commit: I6b07e6cf9b287ba4b5bd6599af4a7412e50b3bdc

Was causing occassional failures for 422 streams due to accessing
elements beyond the extent of the bmi array.

Change-Id: I37ebabf4c01ca84bcd1851428172bdf753805d98