Add buffers for Mask drawing in preparation for bulk API

The next CL will introduce a bulk API between the Painter and
its client code. That CL will uses these buffers to communicate.

Change-Id: I90f78c212db1a3595754e48ab68a41d003ba7003
Reviewed-on: https://skia-review.googlesource.com/c/183920
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Herb Derby <herb@google.com>
2 files changed