add simpler addFrame api (helps with threaded producers)

Change-Id: I458dc2fb59aa32e084b0b03945afd74ff5ee42ae
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/217861
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
Auto-Submit: Mike Reed <reed@google.com>
4 files changed