flip R,G,a for HLG

R, G, and a currently kind of make most sense if you're
looking at the inverse HLG transfer function, but that's
no longer part of our API.  Might as well flip things so
that the one function we do expose is straightforward.

No math really changes here.

For real, this is it as far as PQ/HLG goes.

Bug: chromium:960620
Change-Id: Ia69168e4d3b137ba4d37cf09c714d82dde4ddf92
Reviewed-on: https://skia-review.googlesource.com/c/skcms/+/248550
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
2 files changed