Fifth attempt to land faster rect blur.  Remove unnecessary conditionals from the shader.

approved over in https://codereview.chromium.org/331863006/; reuploading
because it had the wrong base url

BUG=skia:2095
R=bsalomon@google.com
TBR=bsalomon

Author: humper@google.com

Review URL: https://codereview.chromium.org/341543005
4 files changed