Port dering experiment from aom

Mannually cherry-picked:
15791332 Use OD_DIVU for small divisions in temporal_filter.
03122298 Replace divides by small values with multiplies.
9c48eec7 Removing divisions from od_dir_find8()
0950ed82 Merge "Port active map / cyclic refresh fixes to vp10."
efefdad7 Port active map / cyclic refresh fixes to vp10.
1eaf748c Port switch to 9-bit rate cost to aom.
0b1606e7 Only build deringing code when --enable-dering.
e2511e15 Deringing cleanup: don't hardcode the number of levels
8fe5c5d6 Rename dering_in to od_dering_in to sync with Daala
4eb1380d Makes second filters for 45-degree directions horizontal
7f4c3f58 Removes the superblock variance contribution to the threshold
3dc56f93 Simplifying arithmetic by using multiply+shift
cf2aaba9 Return 0 explicitly for OD_ILOG(0).
49ca22aa Use the Daala implementation of OD_ILOG().
85187243 Fix compiler warning in od_dering.c.
485d6a69 Prevent multiple inclusion of odintrin.h.
51b7a998 Adds the Daala deringing filter as experimental

Note that a few of the changes were already in libvpx codebse.

Change-Id: I1c32ee7694e5ad22c98b06ff97737cd792cd88ae