faster RGB->YUV conversion function (~7% speedup)

with a special case for dithering==0., it gets somewhat faster on x86
thanks to inlining.

Also, less macros.

Change-Id: Ic2f2bf6718310743bb40cef2104fa759a073e6d5
cpu: check for _MSC_VER before using msvc inline asm

_M_IX86 will be defined in mingw builds after including windows.h. as
the gcc inline asm is first, this missing check would only have caused
an error if the code was reorganized.

Change-Id: I395679bcfc43e94d308d1ceb0c0fbf932b2c378c