Add flag to hide non-sampling imageshader factories

Bug: skia:11056
Change-Id: Ic79876106c003f6061cbb50ad2f4a4bf4f446231
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/341681
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
21 files changed