Bug 1058900 - [email] pinch/zoom mitigations

Yes, in general it would make sense to unify the double-tap and quantized zoom paths a bit more.  As you surmise, I was trying to leave that code path largely untouched.  Because of the cycling semantics I think the unification would mainly be to have the double-tap logic use lastRequestedScale since it already lines up so much.

I do think at this point, as you say, it likely makes sense to not change this right now.  At least not for v2.0.  I'm happy for us to fine-tune this a little bit more in the future.