with a special case for dithering==0., it gets somewhat faster on x86
thanks to inlining.
Also, less macros.
(cherry picked from commit e2a83d71091abc3e5177ee0ffc4ceb18c7714fab)
Change-Id: Ic2f2bf6718310743bb40cef2104fa759a073e6d5
New function: WebPPictureSmartARGBToYUVA()
This implement smart RGB->YUV conversion.
This is rather undocumented for now, and is triggered using '-pre 4'
preprocessing option.
This is slow-ish and use quite some memory, but should be improvable.
This is somehow a usable beta version.
(cherry picked from commit 3fc4c539aa00c325eba9ef8faec30ec241d71e9c)
Change-Id: Ia50a8c30134e4cab8a7d3eb70aef13ce1f6187a1