skcms_TransferFunction_invert() for PQ/HLG

Absolutely nothing tricky.

This is enough to clean up tf_is_valid().

Bug: chromium:960620
Change-Id: I56c5017fddf167f576879a7d1041455023108c0c
Reviewed-on: https://skia-review.googlesource.com/c/skcms/+/247729
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
2 files changed