Adds a saturated_add function that was on SkNx and used in
SkXfermode_opts, but hadn't been ported to skvx yet.
Removes the Sk4px_opts variants and simplifies some of its functions;
many were already defined skvx.
The largest change is that Sk4px does not extend skvx::byte16, since it
used to extend Sk16b. Now it just has a vector as a data type. This
was necessary so that we could define operators that were typed for
Sk4px and Wide w/o conflicting with the free operators that were
defined for the base skvx types.
Change-Id: I8c667ba86f662ccf07ad85aa32e78abfc0a8c7ae
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/542645
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
767586b330