Cannot double-tap on iOS 13.4+ browsers

[quincylvania]

This is a bizarre bug, one of several apparently introduced in iOS 13.4. I've seen it on mobile Safari, Firefox, and Chrome, so it must be a WebKit issue.

Basically it looks like pointerup/pointerdown events are throttled to about one second for any element with a gesture event listener, even if the event handler doesn't do anything. iD listens for gestures to enable pinch-to-zoom of the map via trackpads. As a result, any double-tap interaction is broken, namely double-tap-to-zoom and double-tap to add vertices to ways. This negates the fixes for #2128 and #2677.

Unless I'm missing something (which is quite possible), our options are these:

• Don't listen for gesture events
• Don't use pointer events for double-tap detection
• Wait for a WebKit patch, if one is forthcoming

[quincylvania]

I realized that we really only need to override gesture events on desktop Safari. Since this bug only affects mobile Safari, I added a check to see if iD is running in mobile webkit or not before adding the event listeners.

User agent sniffing like this is hacky and generally not recommended, but I think it's unavoidable when addressing this kind of browser-specific bug. It's not catastrophic if the detection fails in this case.