SkMipMap::ComputeLevelSize should only cover SkMipMap's levels.

SkMipMap only deals with the levels it generates.
That is to day, it deals with mipmap levels 1-x, not 0-x.
Other functions reflect thing when indexing.
They go from 0 to x-1 (giving the index into SkMipMap's contents).

ComputeLevelSize should also follow that same indexing.

BUG=578304
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2042843005

Review-Url: https://codereview.chromium.org/2042843005
3 files changed