clean-up and slight speed-up in-loop filtering SSE2

* remove some sign-bit flipping
* turn some macro into inline functions
* fix some 'const' in signatures
* clarify the int8/uint8 usage

Change-Id: Ib04459ac34cb280c57579c5d79a5efd2f8d5e99d
Merge "clean-up and slight speed-up in-loop filtering SSE2"
dsp/cpu: add AVX detection

currently unused.
https://software.intel.com/en-us/articles/introduction-to-intel-advanced-vector-extensions

similar checks exist in ffmpeg, libyuv. the visual studio inline asm is
based off of libyuv.

Change-Id: I3e233de3492172434e482607a94b99c617f11aad
Merge "dsp/cpu: add AVX detection"
use decoder's DSP functions for autofilter

-af is now faster (6-7%), since we're using the SSE2 variant
Output is binary the same as before.

Change-Id: If75694594c9501cd486b8f237a810ddcc145cadd
revamp the point-sampling functions by processing a full plane

-nofancy is slower than fancy upsampler, because the latter has SSE2 optim.

Change-Id: Ibf22e5a8ea1de86a54248d4a4ecc63d514c01b88
Merge "revamp the point-sampling functions by processing a full plane"