add isClipEmpty() -- encourage clients to call this rather than checking clipFoo() results

BUG=skia:
R=robertphillips@google.com

Review URL: https://codereview.chromium.org/177073006

git-svn-id: http://skia.googlecode.com/svn/trunk@13568 2bbb7eff-a529-9590-31e7-b0007b416f81
2 files changed