refactor where we wrap clipshaders, in prep for gpu backend

Change-Id: I6040d1fb67911bbfdefd5f1ff968e7d11323763d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/276281
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
6 files changed