add explicit API to know if the writebuffer is using its external storage

To help with https://skia-review.googlesource.com/c/skia/+/114262

Bug: skia:
Change-Id: I83fcccdec0dbaef28d0e557ea6fb9bd0ed59ed44
Reviewed-on: https://skia-review.googlesource.com/114284
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Mike Reed <reed@google.com>
4 files changed