Refactor API for mipmap-builder,

now the builder returns the new image.

Change-Id: Ie56256390b96d3fdbe39f89784276947047df656
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/316442
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
5 files changed