Improve detection of color space transfer fns

BUG=skia:

Change-Id: I744af0efd4d48a8932b834092ed2dbad13008c1d
Reviewed-on: https://skia-review.googlesource.com/6556
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Matt Sarett <msarett@google.com>
3 files changed