Hide SkImageFilter subclasses in CPP files

Bug: skia:9280
Change-Id: Ice3624467f5e643f9d3d81da72d77911eeac963e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/230198
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
38 files changed