[RequestAnimationFrame] Processing model defined

I've added a formal definition of the requestAnimationFrame processing
model, check it out here:
http://dvcs.w3.org/hg/webperf/raw-file/8ca96303d8f5/specs/RequestAnimationFrame/Overview.html.
 I believe this closes issues 1 and 5 and have marked them as such on the w3
issue tracker.  It defers to the page visibility specification's definition
of "hidden" to decide whether or not to surpress callbacks, with the
assumption that the page visibility specification will figure out what the
correct behavior is for things like print media, non-visual user agents, and
the like.

The one point of potential interest or controversy is the expected callback
behavior for background tabs.  The processing model I've specified says that
callbacks do not fire at all when a document is hidden (in the page
visibility sense of "hidden"), which is consistent with what we've
implemented in WebKit and what I understand is Microsoft's preference, but
not consistent with the Firefox behavior.  We've discussed this a few times
on the list before and absent any new data I think that not firing callbacks
is a better default behavior for users since it uses the least CPU for
background content.  We can revisit this if we get any new data, for example
if we find evidence of compatibility issues with suppressing the callbacks
completely.

- James