Add smart RGB->YUV conversion option -pre 4

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.

Change-Id: Ia50a8c30134e4cab8a7d3eb70aef13ce1f6187a1