calculate root bounds directly

This is a long-standing TODO.  No reason to ask
the BBH subclasses to calculate the root bounds
when we can simply union them up ourselves.

Change-Id: I9dbd883c43247400e4e9d56c74d4203d34f698e1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/270276
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
6 files changed