Add support for child effects on SkRuntimeBlender.

This will allow runtime blenders to leverage existing color filters or
shaders when computing a blend.

Change-Id: I743d5fc6d83a92cd190cb6bfd1c79789d8be2089
Bug: skia:12249
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/430658
Commit-Queue: John Stiles <johnstiles@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
5 files changed