Previously, the projected positions of chroma pixels would effectively
undergo double rounding, since we round both when calculating x4 / y4
and when calculating the filter index. Further, the two roundings
were different: x4 / y4 used ROUND_POWER_OF_TWO_SIGNED, whereas
the filter index uses ROUND_POWER_OF_TWO.

It is slightly more accurate (and faster) to replace the first
rounding by a shift; this is motivated by the fact that
ROUND_POWER_OF_TWO(x >> a, b) == ROUND_POWER_OF_TWO(x, a + b)

Change-Id: Ia52b05745168d0aeb05f0af4c75ff33eee791d82