Rename SkBitmapController to SkMipmapAccessor

Change-Id: Idf9543db44c2518a9fab7c90ca52f3b1c97937a0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/341677
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
6 files changed