When the "dy" variable is between -1 and 0 (base2 == -1)
we should interpolate between the upper-left(-1,-1) and left(-1,0)
samples, instead of just taking the left sample.

Coding gain improves a bit with this change.

BUG=aomedia:357

Change-Id: If520f71b22115e79120499c21f8d6925c1fa24be