Makefile.vc: add rudimentary avx2 support

similar to makefile.unix:
> nmake /f Makefile.vc CFG=release-static HAVE_AVX2=1

from the msdn:
The /arch:AVX2 option and __AVX2__ macro were introduced in Visual
Studio 2013 Update 2, version 12.0.34567.1
(Update 2, version 12.0.30501.00 seems to work)

Change-Id: I649ee47c9fdc399fc71a8ac8464728608d9b6412
Merge "Makefile.vc: add rudimentary avx2 support"
lossy decoding: correct alpha-rescaling for YUVA format

The luminance needs to be pre- and post- multiplied by
the alpha value in case of rescaling, for proper averaging.

Also:
- removed util/alpha_processing and moved it to dsp/
- removed WebPInitPremultiply() which was mostly useless
and merged it with the new function WebPInitAlphaProcessing()

Change-Id: If089cefd4ec53f6880a791c476fb1c7f7c5a8e60