Be smarter about Mapbox layer creation for GeoJSON sources

[goldpbear]

Starting with changes introduced in #956, we treat GeoJSON layers as combinations of Mapbox fill, symbol, and line layers, anticipating GeoJSON sources that might have any combination of Polygon, Point, and LineString geometry in them.

Furthermore, we allow GeoJSON sources to be styled with zoom style rules in the config, creating the need for a set of fill, symbol, and line layers for each style condition. So the total number of layers created is 3 multiplied by the number of style conditions.

For GeoJSON sources that don't contain multiple geometry types this approach creates a lot of "dead" layers whose filters will never apply to any geometry. This might be a performance bottleneck, and we might want to think about a more intelligent way of building layers to support GeoJSON sources.