similar to '* const', __restrict needs to be included in the
declaration to avoid warnings like:
src\dsp\alpha_processing.c(429): warning C4028: formal parameter 1
different from declaration
this change also moves WEBP_RESTRICT to dsp.h to avoid a circular
dependency between it and utils.h which already includes dsp.h
Change-Id: Ib070d358a372e76fae4be5445ab288940b9baac0
DispatchAlpha* functions are hard to speed up, compared to SSE2.
ExtractAlpha sees a ~15% speed-up though.
Change-Id: I8715c2defecbc832f469eed7e6ffd012146b52de