addLayer with inline source mutates its input

[jfirebaugh]

addLayer with an inline source will mutate the argument, replacing the source object with the layer's id. Runtime styling APIs should never mutate their input, as users may want to reuse the input, or otherwise expect it not to change outside their control.

var source = { ... };
var layer = { ..., source: source };
map.addLayer(layer);
assert(source === layer.source); // should pass