Composite Provider for UTFGrid layers
https://github.com/mapbox/utfgrid-spec/blob/master/1.2/utfgrid.md
 
Combines multiple UTFGrid layers to create a single result.
The given layers will be added to the result in the order they are given.
Therefore the last one will have the highest priority.
 
Sample configuration:
        "provider":
        {
                "class": "TileStache.Goodies.Providers.UtfGridComposite:Provider",
                "kwargs":
                {
                        "stack":
                        [
                                { "layer_id": "layer1", "src": "my_utf_layer1", "wrapper": "grid" },
                                { "layer_id": "layer2", "src": "my_utf_layer2", "wrapper": "grid" }
                        ],
                        "layer_id": "l",
                        "wrapper": "grid"
                }
        }
 
stack: list of layers (and properties) to composite together
        layer_id: an id attribute that will be added to each json data object for this layer: { "layer_id": "layer1", "name": "blah", "address": "something"}
        src: layer name of the layer to composite
        wrapper: the wrapper definition of this layer if there is one (so we can remove it)
layer_id: the key for the layer_id attribute that is added to each data object: { "l": "layer1", ...}
wrapper: wrapper to add to the resulting utfgrid "WRAPPER({...})". Usually "grid"
 
if layer_id is not set in the layer or the provider config then it will not be set on data objects

Modules
		TileStache json

Classes
		Provider SaveableResponse   class Provider       Methods defined here: __init__(self, layer, stack, layer_id=None, wrapper=None) addLayer(self, resultGrid, gridKeys, gridData, layerDef, coord) decodeId(self, id) encodeId(self, id) getTypeByExtension(self, extension) Get mime-type and format by file extension. This only accepts "json". renderTile(self, width, height, srs, coord) writeResult(self, resultGrid, gridKeys, gridData)   class SaveableResponse     Wrapper class for JSON response that makes it behave like a PIL.Image object. TileStache.getTile() expects to be able to save one of these to a buffer.     Methods defined here: __init__(self, content) save(self, out, format)