move SkIPoint16 to private header

Bug: skia:
Change-Id: Ib8045ac7cc24a44c4b70e73153c6faf098730b63
Reviewed-on: https://skia-review.googlesource.com/118721
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Cary Clark <caryclark@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
10 files changed