[sksg] Hide some implementation methods from public Transform API

TBR=
Change-Id: I29ad15e83a64e6d9f13cd601379c6b624cba4ab5
Reviewed-on: https://skia-review.googlesource.com/c/182817
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
4 files changed