Add per-vertex coverage field, use in AA rects when alpha tweak is not valid, add relevant sample/gm

Review URL: http://codereview.appspot.com/5181044/



git-svn-id: http://skia.googlecode.com/svn/trunk@2440 2bbb7eff-a529-9590-31e7-b0007b416f81
12 files changed