Refactor SkMipmap::Build

See https://issues.skia.org/issues/307987911

Change-Id: I8767be465fe2cac4ca1eaebf22900615e3a9fff1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/768279
Commit-Queue: Mike Reed <mike@reedtribe.org>
Reviewed-by: Brian Osman <brianosman@google.com>
7 files changed