Add blend optimization helpers and use to convert rect draws to clears.

R=robertphillips@google.com, jvanverth@google.com, reed@google.com

Author: bsalomon@google.com

Review URL: https://chromiumcodereview.appspot.com/21877006

git-svn-id: http://skia.googlecode.com/svn/trunk/include@10537 2bbb7eff-a529-9590-31e7-b0007b416f81
4 files changed