Move VectorXform into the wangs_formula namespace

This class is only meant to be used with Wang's formulas, so it
belongs in their name namespace.

Bug: skia:12524
Change-Id: Ib1667781b947a7f052af103b7bf9084d09a690de
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/461567
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
7 files changed