Hide path tessellation shaders in private files

Replaces the public class definitions with factory methods. Separates
the class definitions into their own separate files.

Bug: skia:10419
Change-Id: I574d920d5a3d0dc98fa5eb231c9b510e04aebf78
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/413796
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
9 files changed