This fixes the bicubic image filter GM on the GPU, which otherwise draws garbage outside the filtered region. It also moves us closer to unifying the signatures of SkImageFilter::onFilterImage() and SkImageFilter::filterImageGPU().
Review URL: https://codereview.appspot.com/7180048
git-svn-id: http://skia.googlecode.com/svn/trunk@7467 2bbb7eff-a529-9590-31e7-b0007b416f81