fix: support pinch-to-zoom behavior

[rpearce]

Description

In theory, this fixes #436, and is a response to #472, but I'm not opposed to having that as an option, anyway.

The goal with this work is to allow users to pinch-to-zoom in on RMIZ-zoomed images and pan around without being unzoomed (not a nice experience). Once they return the zoom to the normal browser zoom level, then swiping will close the image.

NOTE: I still need to test this on an Android phone and an iPad.

Testing

1. Clone the repo and checkout this branch: fix/pinch-zoom
2. npm i --no-save && npm start
3. Get your local IP address by running this (should work...)

   ifconfig | grep -Eo 'inet (addr:)?([0-9]*\.){3}[0-9]*' | grep -Eo '([0-9]*\.){3}[0-9]*' | grep -v '127.0.0.1'
   

4. On a mobile device in the browser, go to the first IP address above at port 6006; e.g., http://192.168.1.152:6006
5. Open the "Sidebar" and find the example you want to test
6. Open the "Canvas" view
7. Click the 4 arrows pointing in different directions
   
8. Tap on examples and test pinch-to-zoom behavior alongside single finger swipe-to-scroll behavior