Store SkColorSpaceXform gamma LUTs in a malloced field

Store SkColorSpaceXform gamma LUTs in a malloced field

In order of likelihood:
(1) Tables are never used, since gamma is recognized and named.
(2) Only use one table, since all three gammas are the same.
(3) Actually need three tables.

No reason to waste a bunch of space for these tables on
SkColorSpaceXform, when it will likely be unused.

This will be more efficient in lots of cases, but is particularly
useful when the client really only wants a gamut xform.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2336913005

Review-Url: https://codereview.chromium.org/2336913005