[e] (0) Clarify what we mean by 'poster frame' of an animation.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=21296
Affected topics: Canvas, DOM APIs

git-svn-id: http://svn.whatwg.org/webapps@7847 340c8d12-0b0e-0410-8428-c7bf67bfef74
(cherry picked from commit 5eb5e2bc082a857424d4a728fe6599a7f1a83515)

HTML WG Bug# https://www.w3.org/Bugs/Public/show_bug.cgi?id=21872