[sksg] Clean up public headers

Change-Id: I0a6627f9b8d47e51c82c89ff1df75c3add8e8035
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/267919
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
3 files changed