[skif] Add ::Empty() convenience factory
I chose to make it public since it seems relatively harmless, but this
is mostly used to simplify the internsl of the source filters that
otherwise all had to handle cases where their sources were null.
Additionally, this will let me delete a number of custom image filter
subclasses for "fail" testing in unit tests, that can be updated to
just use ::Empty() instead.
Bug: b/283548627
Change-Id: I5f635725b897a205aff2ac64583bf077d2ae2b8f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/743657
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
9 files changed