Add GrVx.h

Adds grvx, Ganesh's addendum to skvx. Here we introduce familiar names
and operations from GPU languages, as well as functions that are
approximate and/or have LSB differences from platform to platform.

The initial implementation has: fast_fma, fast_acos, and
fast_angle_between_vectors. When a function is approximate, its error
range is well documented and tested.

Also establishes GrWangsFormula as the first user of grvx.

Bug: skia:10419
Change-Id: Id0682599cf9c0303eff386095afc3ef9f3a7fa1b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/330119
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
8 files changed