Review updating strokes with incoming events

[adamwulf]

In MMDrawModel near line 56:

            // TODO: It's possible for an update to come in for a previous stroke
            // after a new stroke has aleady been started. In that case, the
            // for loop below will never be run to find the correct stroke,
            // and the isSameTouchAsEvent: will also fail, causing the event
            // to be lost.

Also in [MMDrawnStroke addEvent:], all of the elements events were re-mapped into _eventIdToSegment instead of only elementEvents. Verify that using elementEvents is enough.

[adamwulf]

I think the correct strategy is to check for existing stroke's waitingForEvent: immediately if either !strokeForEvent or [[strokeForEvent event] isSameTouchAsEvent:event]. This way a stroke is found if the current active stroke isn't a match as well.