Add an option to SkCanvas to turn on path
ops when combining clips.
Allow Op() to use one of the input paths
as an output path.
Fix a bug in Op() when the minuend is empty
and the subtrahend is not (for difference).
Change the build to allow core to depend on pathops.
Review URL: https://codereview.chromium.org/14474002
git-svn-id: http://skia.googlecode.com/svn/trunk@8855 2bbb7eff-a529-9590-31e7-b0007b416f81
