Use geometric mean when selecting a mipmap scale

Workaround for anisotropic mipmap quality issues.

R=reed@google.com,robertphillips@google.com
BUG=skia:4863,chromium:586894
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1697423002

Review URL: https://codereview.chromium.org/1697423002
1 file changed