make SkBBH a public interface

I don't necessarily like this long term, but in the short term Flutter
would like to record pictures using their own type separate from
SkRTree.  This makes SkBBoxHierarchy public, and converts it to use
other public types (SkTDArray -> vector).

Change-Id: I29c5ef9da7d641d8f4ba18522b168ddf7cefe84f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/270387
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Reed <reed@google.com>
17 files changed