[skif] Add FilterResult::Builder and update SkMergeImageFilter to use it

Bug: skia:9282
Bug: b/263133201
Bug: b/263133780
Change-Id: I568eb34255309d18c9699443727a5f8661d4cd39
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/697478
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
3 files changed