Reorganize and hide aa clip impl into .cpp file

Bug: skia:10205
Change-Id: Ia2dd9b99f5282a46aaeaf8eac54c9f6bfd583fc6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/445617
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
5 files changed