Add convenient "xyzw" accessors and swizzles to skvx

Change-Id: Ic300285d10679a4e34190ab7b6b08bd1f6d80330
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/454309
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
3 files changed