element. This filter has two inputs, since normal blending can't be used. The
GPU side uses two filter stages to accomplish this: one to sample the
background, and one to sample the foreground and blend it.
Review URL: https://codereview.appspot.com/6463081/
git-svn-id: http://skia.googlecode.com/svn/trunk@5231 2bbb7eff-a529-9590-31e7-b0007b416f81