move parts of SkPoint to SkPointPriv

Move specialized SkPoint methods to SkPointPriv.
Use constexpr and inline initialization where possible.

R=reed@google.com,bsalomon@google.com
Bug: skia: 6898
Change-Id: I01ec5186f010f2dc80c068c70d9cc352f3221338
Reviewed-on: https://skia-review.googlesource.com/68700
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Ravi Mistry <rmistry@google.com>
39 files changed