[whatwg] remove resetClip from the Canvas 2D spec

[Simon Sarris]

> Good point, I think part of the problem has to do with the fact that save
> is non-selective (saves all of the state).

Yes, since save() and restore() save and restore everything, it creates the
side effect of needing to set ctx.font/fillStyle/strokeStyle more often
than otherwise, which are slow to set, probably because of some CSS parser
activity, but I'm not wise enough to know.

If there was merely a way to save and restore the context, or perhaps some
other subset of state, that would probably work nicely too.