speed up A8 by creating a new entry-point in SkDraw that blits the path's coverage directly into an A8 target, regardless of the previous pixel values.

R=bsalomon@google.com, mtklein@google.com

Review URL: https://codereview.chromium.org/56453002

git-svn-id: http://skia.googlecode.com/svn/trunk@12118 2bbb7eff-a529-9590-31e7-b0007b416f81
11 files changed