Enable testing of native pinch to zoom in Firefox

Update

Native support has landed in 83.0.

It now works well with history navigation, placement of context menus and web content like Wikipedia article previews.

Scrollable content within a zoomed-in page works, but still has twitchy scroll bars.

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This finally works natively for me on a Macbook Air. No more need for CSS wizardry from extensions (I used Multi-touch Zoom up until now, so thanks to the author).

Set apz.allow_zooming to true on the about:config page in Firefox Nightly 71.0a1.

I don't know for how long this option has been available [Update: since earlier this year apparently], but I found it just now by following the trail from 688990 and ending up applying the steps to reproduce (STR) in 1556556.

Scroll behavior is not perfect while zoomed in, but I think it's still worth it in order to reduce the layout problems that could result from using the extension on some webpages (e.g. Reddit). Zoom out, reposition the cursor and zoom in again is an acceptable workaround until the feature lands.

The responsiveness is also hugely improved compared to the extension. There is no more delay between the pinch and the zoom, and it's now also possible to initiate a pinch while the page is scrolling, which saves time from first having to stop the inertial scrolling -- something that was always necessary with the extension.