Provide a context pointer for every skcms op.

Previously, some ops would include a context pointer and others
would not; the stage was responsible for incrementing the context
pointer if necessary. Now, ops which do not require a context
pointer insert a null into the context array.

This improves performance slightly on my M1 and makes the code
more amenable to tail-calling.

Change-Id: I0d9bbcd19124d0a1d8a2ebaef20633969f16978a
Reviewed-on: https://skia-review.googlesource.com/c/skcms/+/771480
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
2 files changed