Pinch-to-zoom gesture should pan the map when both fingers move

Nobody has claimed this yet.

**mapbox-gl-js version**: v1.0.0
**browser**: iOS Safari / Android Chrome / Edge on Mobile

Capturing https://github.com/mapbox/mapbox-gl-js/pull/8100#issuecomment-499933976 by @1ec5:

> does GL JS currently address the case of a two-finger pinch-to-zoom gesture where the averaged point between the fingers moves (a “wandering zoom”)? On a touchscreen, it’s quite difficult to keep that centerpoint stationary as you pinch your fingers or spread them apart. To maintain a responsive feel, it’s necessary to pan at the same time as zooming (though perhaps some use cases may call for a more precise gesture handler that locks the center while zooming).

### Steps to Trigger Behavior

1. Start zooming the map on a touch device
2. Still touching the map with two fingers, try panning the map

### Expected Behavior

The map moves so that the location between fingers persist through the whole gesture. For comparison, Leaflet and various other maps APIs do this.

### Actual Behavior

The map doesn't move.

### Link to Demonstration

https://docs.mapbox.com/mapbox-gl-js/example/simple-map/

Reproduce the behavior on a touch device using the linked simple-map demonstration and the two-finger steps in the issue. Compare the observed gesture with the expected behavior: the map should pan while zooming so the location between the fingers remains consistent throughout the gesture.

Written by the indexing model from the issue text.