Move ICCTag struct and GetTag functions to skcms_internal.h

Clients can't do anything with this API, so let's avoid tempting them.

Change-Id: I647f40bd31be665648ffcdea1a89e6381721431f
Reviewed-on: https://skia-review.googlesource.com/c/skcms/+/253936
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
3 files changed