Reload source tiles when GeoJSON data is mutated

[jfirebaugh]

Followup from #6201.

[jfirebaugh]

GeoJSON tile reloading will be similar to reloading for style changes in #6201, but also different.

The pathway for reloading for style changes is:

1. A layout property or layer filter is changed
2. Triggering LayerObserver::onLayer{Filter,Visibility}Changed
3. Triggering Style::onLayer{Filter,Visibility}Changed
4. Style queues the layer source ID in updateBatch.sourceIDs and triggers style::Observer::onUpdate with Update::Layout
5. Map::Impl::onUpdate adds Update::Layout and triggers an asyncUpdate signal (if not already triggered)
7. The signal is processed; Map::Impl::update is called
8. Triggering Style::relayout
9. Loop over updateBatch.sourceIDs, calling Source::Impl::reloadTiles for each
10. Source::Impl::reloadTiles loops over tiles, calling Tile::redoLayout for each
11. Which is overridden in GeometryTile::redoLayout to trigger the workers to reprocess with a new copy of the style layers

All this only handles changes to style layers. It's fairly complex in order to support batching multiple updates into a single redoLayout operation. For GeoJSON data changes, maybe we can get away without batching, and instead just add a loop over tiles in GeoJSONSource::Impl::setGeoJSON, calling GeometryTile::setData on each one with the new data.

[tobrun]

I have been working with @mollymerp to get this feature in.
We were able to get the the following working:

For GeoJSON data changes, maybe we can get away without batching, and instead just add a loop over tiles in GeoJSONSource::Impl::setGeoJSON, calling GeometryTile::setData on each one with the new data.

An example of this can be seen in:

Which is the equivalent of the GL-JS example here.

[tobrun]

@jfirebaugh would you like to see the improved version happening?
(implementing the observer approach with batching)

[jfirebaugh]

No, I think we're good here unless we encounter a concrete performance issue.