Revert "flop back to powf() for TF invert"

This reverts commit a7974d9f65a55e8796d491b7b1841aa41391d373.

Reason for revert: Nevermind.

Original change's description:
> flop back to powf() for TF invert
> 
> Seems to remove my diffs (however weird) locally.
> 
> Change-Id: Ic32d22858b83c1d70aaa38d22669342b7f3aeb24
> Reviewed-on: https://skia-review.googlesource.com/118982
> Commit-Queue: Brian Osman <brianosman@google.com>
> Reviewed-by: Brian Osman <brianosman@google.com>

TBR=mtklein@chromium.org,brianosman@google.com

Change-Id: Id9af3aa2a7ebd0eedf494b71d3055f09fd65663c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/119000
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
2 files changed