add skcms_ApproximatelyEqualProfiles() fastpath

We're not checking exact equality, and doing so wins back a lot of the
performance lost when switching SkCodec to use skcms.

I've noted a few other places that look ripe for improvement.

Bug: skia:8278
Change-Id: Idcd17736b3928dda47ecd5e65c8ce05718974b14
Reviewed-on: https://skia-review.googlesource.com/150243
Commit-Queue: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
2 files changed