`data` event from style mutations should be fired synchronously

Nobody has claimed this yet.

Currently, if you mutate the style via `setPaintProperty` or some other runtime styling API, the corresponding `data` event (with `dataType` property equal to `style`) is not fired until the subsequent repaint, which is asynchronous. I think this event should be fired immediately instead:

* The change is observable with respect to `getPaintProperty` or comparable accessor immediately, and it seems like the event should fire as soon as the change becomes observable.
* It's necessary for #2741: `Map` will use the `data` event as the consistent trigger for a new frame, rather than wrapping each `Style` method and calling `_update(true)` manually.
* The current behavior is causing each style mutation to trigger two repaints: one from the manual `_update(true)`, and a second one from the `data` event that's triggered during the first repaint.

Start by tracing setPaintProperty and the other runtime styling APIs through the Style and Map update paths, then follow where the style data event is dispatched during repaint. Confirm that the event is emitted when the mutation becomes observable and that one style mutation no longer causes two repaints.

Written by the indexing model from the issue text.