Change it from transparent white to transparent black, which matches
the transparent color assumed in Webp dispose-to-background method.
Also pre-multiply background colors before comparison in anim_diff,
just as what is done with regular pixel values.
Change-Id: I5a790522df21619c666ce499f73e42294ed276f2
though visible WebPCopyPlane & WebPCopyPixels are not part of the public
api; avoid using a private header within this public module.
Change-Id: I5c8615fcc07090ffaa8933b00af418d8431936eb