Prep for tailcall-based implementation.

This CL lands the portions of http://review.skia.org/771479 that can
be patched in safely without actually switching to a tailcall-based
implementation. (There is a lot of prepwork that needs to happen
first, so the actual switch can't happen for a while, so this limits
the amount of code that will need to be rebased.)

Change-Id: I4ca40980ee8bab0cffecc7d2dd4756371e8b33a0
Reviewed-on: https://skia-review.googlesource.com/c/skcms/+/773234
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
4 files changed