[agiow] (0) First draft of a way to do canvas in workers.
Affected topics: Canvas, DOM APIs, HTML, Rendering, Security, Web Workers, Workers

git-svn-id: http://svn.whatwg.org/webapps@7509 340c8d12-0b0e-0410-8428-c7bf67bfef74
(cherry picked from commit 35cb29a4c417cde6ad99c16b2d38ac220e2e7d47)

NOTE: This also introduces some HD functionality, so can't work without
the feature/whatwg_canvas branch.