Missing sourcedata event after sourcedataloading events

Nobody has claimed this yet.

**mapbox-gl-js version**: 1.13.0

**browser**: Firefox

### Steps to Trigger Behavior

Flick the map some distance away to trigger a `sourcedataloading` event, then quickly flick it back to the start.
Although the `sourcedataloading` event has fired, no `sourcedata` event follows. The flick has to be sort of fast, though it seems to depend on network speed.

### Link to Demonstration

jsfiddle: https://jsfiddle.net/o6da71nq/
gif demonstration: https://drive.google.com/file/d/1EBQbggpFSHTlWx1swHqxkBeAhL7dGT42/view?usp=sharing

### Expected Behavior

A `sourcedata` (or `error`) event should follow the `sourcedataloading` event(s), as described in `sourcedataloading`'s spec:
>Fired when one of the map's sources begins loading or changing asyncronously. *All sourcedataloading events are followed by a sourcedata or error event.*

### Actual Behavior

Neither a `sourcedata` nor `error` event is fired after the `sourcedataloading` event(s)

### Potential Workaround

For any other users experiencing this, one workaround seems to be to listen to the `idle` event as a backup. If the `idle` event fires while you are expecting a `sourcedata` event, you can fallback to some behavior.

Reproduce the sequence in the linked jsfiddle: move the map to trigger sourcedataloading, then quickly return to the starting position. Trace the sourcedataloading, sourcedata, and error event handling; done means every sourcedataloading event is followed by sourcedata or error, including during rapid movement.

Written by the indexing model from the issue text.