Adding anisotropic mipmap levels to SkMipMap.

Adding 1x2, 1x3, 2x1, 3x1 filters to SkMipMap and enabling SkMipMap to generate anisotropic mip levels.

BUG=590804
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1750303002

Review URL: https://codereview.chromium.org/1750303002
2 files changed