Figure out if layer filtering can be improved in general

[mattdelsordo]

At the time of writing, layers are filtered by loading in Mapnik XML as a string, compiling it to JSON, modifying the JSON, and then compiling it back to XML. This was done because Mapnik's layers aren't editable once added to a map object, and layers are only added when maps are generated from XML, so the layers need to change before the map is created (e.g. before there's even an actual JS data structure representing the map). There seems like there should be a more efficient way to do this.