migurski/TileStache — pull request #85 by alno (2012-09-27 16:50:05+00:00)
Title: Fixing mapnik grid usage: different grids for different layers

As described in https://github.com/mapnik/mapnik/issues/1325 grid object is supposed to be rendered only once.

Multiple rendering produces two weird errors:
* As described in mapnik issue, all attributes of previous layers are included in subsequent layers
* Only attributes of first layer are encoded in resulting grid

So, as in old method every layer should be rendered to own grid, with subsequent merge of all grids.
Also, it reduces some code duplication when layer_id_key = true/false.