Anisotropic mipmap fixes

1) when selecting a level scale, use max(scaleX, scaleY) instead of
  current sqrt(scaleX * scaleY)

2) track and apply non-uniform fixup scales

R=reed@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1617183004

Review URL: https://codereview.chromium.org/1617183004
3 files changed