Use the Daala implementation of OD_ILOG().

The Daala implementation of OD_ILOG(x) computes the number of bits
 necessary to represent the unsigned integer x in two's compliment.
We adjust the computation in od_dering.c to better approximate the
 log base 2 and match the previous OD_ILOG() definition.
This commit makes no change to the bitstream.

Change-Id: Ib75c2f47dca7027b3cfc40b110f2d493f9690c9c