Features that span across quadkeys disappear and then come back

I'm going to put this down as a `wontfix` for now, because the flash is pretty minor, and it's pretty hard to fix, but wanted to create an issue to document this in case someone else sees some features flashing out and then back in while panning and is wondering what's going on.

When we fetch data for a quadkey, it gives us all features that intersect with that quadkey, including some roads for eg. that are really long. When you pan out of that quadkey, it starts a request for your current quadkeys and, to keep the geojson on the map lighter, removes elements that belonged to a quadkey that's no longer part of the viewport. When the feature appears again in the request for the new quadkey, that feature re-appears.

To cause features that are in the data key of a quadkey no longer in view but that still appear on the screen not to be removed when a user pans will involve inspecting the geometry of each feature on the map - I don't think it's worth the trade-off here. But just wanted to document this in case it comes up.