Add an SSE4.1 lowbd version of the self-guided filter for
loop-restoration, and apply some optimizations to the C
version.

Approximate times per 128x128 / 256x256 tile on the machine
this was developed on:
Previous C:  620us / 2800us
Optimized C: 500us / 2200us ( 24% /  27% faster)
SSE4.1:      147us / 600us  (320% / 370% faster)

Change-Id: I23ff5a5482a191aeb06f9d1f767a9f036bb357fe