[giow] (0) Define an API to clear all the hit regions, for authors who don't need to use clearRect() since everything they paint is opaque.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=26404
Affected topics: Canvas

git-svn-id: http://svn.whatwg.org/webapps@8747 340c8d12-0b0e-0410-8428-c7bf67bfef74