replace gamma_correct with gamut/transfer_fn in DM

gamma_correct as is (really meaning, linear blending) isn't super
useful, in that it doesn't distinguish gamuts or transfer functions
except between identity and non-identity.

This replaces it with fields that describe the gamut and TF.

Change-Id: Ied08c2df537be61ee1903ef6cc279991326271a2
Reviewed-on: https://skia-review.googlesource.com/c/191573
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Stephan Altmueller <stephana@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
3 files changed