Add optional extent parameter to OGR datasource plugin

This pull request addresses issues https://github.com/mapnik/mapnik/issues/2260 and https://github.com/mapnik/mapnik/issues/1029.  Not all OGR layer objects return an extent.  When this happens, the OGR datasource plugin in mapnik reports a (0,0,0,0) extent and does not return any features.   This change adds an optional extent parameter to the OGR datasource plugin which will allow users to manually set the datasource envelope in cases where OGR does not report an extent.