replace Sk4px with SkVx in SkBlitter_ARGB32

This is a 1:1 transliteration, with the skvx code more explicit about
what's going on, the old Sk4px code more abstracted and specificially
designed to work with unorm8 types.  It's not as terse, but I think I've
got it equally or more clear?

I haven't changed anything here, but I think it should be pretty clear
how to generalize this up to say, 8 pixels at a time?

Writing this CL has got me pretty strongly considering adding explicit
bit-pun constructors to the skvx::Vec types, given how often I'm finding
them to be useful, particularly between uint32 and 4x uint8.

Change-Id: Ie62340e571258fc82176ae3d6605b1f96695f90e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/207722
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
1 file changed