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>
2 files changed