improve integer version of filter

the lookup table is based on floating point calculations (see source)

by moving the *3 before the downshift and adding the rounding bit, the
delta (LUT - integer) goes from:
______________________________________
__ 1__ 1______________________________
__ 1__ 1______________________________
____ 1______ 1________________________
____ 1 2__ 2 1________________________
______ 1 1 2__ 2__ 2__ 2 1 1__________
________ 1 1 2 2__ 1 2 3 1 2__ 2__ 2__
to:
__-1__-1______________________________
______________________________________
____-1______-1________________________
______________________________________
________-1______________-1____________
______________________________________

it's important to be able to use the integer version because the LUT
more or less precludes SIMD optimizations

Change-Id: I45a81127dc7b72a06fba951649135d9d918386c0