Implement source.filter from the MapBox styleJSON spec

[timbtimbtimb]

New Feature

Add filtering at the source level without having to set the data attribute. This already exists as source.filter in the MapBox style spec.

Why

I would love to be able to filter sources without having to set the data attribute, and be able to filter before clustering (which layer-level filtering doesn't allow). mapbox-gl already implements this and it's super convenient.

Thank you :)

[timbtimbtimb]

I'd like to add more context and my particular usecase:

I use rnmapbox/maps and I have a 16000 points FeatureCollection<Points> object on which I perform filtering. I cannot use layer-level filtering because it ignore clusters, so I have to filter my Javascript object and pass it over the bridge again, at a huge performance loss as you can imagine. Using the far more powerful MapBox C++ filtering would be so much more elegant and much faster.