Hide SkMatrixTransformImageFilter in cpp file

Bug: skia:9280
Change-Id: I5effaa41fa9f805ccc7c254f226ef8963a04a5f4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/569335
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
11 files changed