tweak SkVx to play nicely with others

Was starting to use this and ran into a few problems with clashing
symbols, namely SI and cast().  Seemed simple enough to not use SI,
and to move all the free-standing types into skvx: skvx::cast,
skvx::shuffle, etc.

Change-Id: Ia5d8ef6d0ae5375bf80d76be88d16f0c9cde56e7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/207340
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
2 files changed