expose clear way to create default mips

Change-Id: Id533b71d56fc44d8f86db24c5c4e9131d57479a5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/316716
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
3 files changed