memory_dataset mostly broken (patch needed for simple data-point display)

[lwu]

In order to implement a simple point_datasource class, one may inherit publicly from memory_datasource.

For example, like so: http://gist.github.com/6150

however, this code relies on the mapnik::memory_datasource to actually do what it's supposed to, that is, provide a query method via memory_datasource::features(const query& q).

Feature query is implemented in terms of mapnik::memory_featureset(), but the next() iterator functionality was commented out a year ago, most likely due to geometric data structure re-structuring.

The patch should be attached. This has been tested to work, and to support the use case of point_datasource (as mentioned previously).

[lwu]

Patch to re-enable memory_datasource fn (specifically commented out iterator code)

[springmeyer]

Does the patch with Ticket #113 supersede this one?