add SkVx helpers for working with unorm8

These replicate the base logic of Sk4px::Wide::div255() and
Sk4px::approxMulDiv255(), and will come in handy replacing them.

No platform specializations yet... want to remind myself what
codegen they get from these vanilla versions first, and then
I'll fill in the platform specific stuff as needed.  The tests
should cover everything pretty exhaustively.

Change-Id: I5854d1bc0902a85cbb2351f669c4da7cc31a8775
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/207683
Commit-Queue: Mike Klein <mtklein@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
2 files changed