With the sad functions, and hopefully the variance functions soon,
moving to the vpx_dsp location, place the defines used in the
reference C code in a common location.
Change-Id: I4c8ce7778eb38a0a3ee674d2f1c488eda01cfeca
Create a new component, vpx_dsp, for code that can be shared
between codecs. Move the SAD code into the component.
This reduces the size of vpxenc/dec by 36k on x86_64 builds.
Change-Id: I73f837ddaecac6b350bf757af0cfe19c4ab9327a