Replaces division with multiplication in self-guided
filter.
The guided filter requires computation of:
n^2.s^2/(n^2.s^2 + n^2.e).
This is now implemented by computation of n^2.s^2/n^2.e followed
by using a lookup table for the function f(x) = x/(x+1).
To compute n^2.s^2/n^2.e, we use an integer multiplication based
implementation which becomes feasible since n^2.e can only
take a few values and their corresponding multipliers can be
pre-computed.
There is also another divison by n, that is also integerized.
Change-Id: Id7b81bbafead0b8f04a1853ec69b9dec423bb66a