remove inv()

inv(x) -> 1-x

Change-Id: If64591f4f3ce15eb487222f052fc4bd1bac97977
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/280316
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Reed <reed@google.com>
3 files changed