Avoid hitting length limit for GET requests
When serving WMS for stylesheets with many layers, and layers with long names, its easy to hit the limit for length of GET requests for a given browser.
A "meta layers" hack was added to work around this problem for a haiti deployment of the ogcserver: https://github.com/mapnik/OGCServer/blob/master/ogcserver/common.py#L384 and this needs to be removed and proper support added.
The natural thing to do would be to follow the WMS spec, and allow Layers to contain other Layers (wms 1.1.1 spec: 7.1.4.5.2). But the support must be added to mapnik first.
Initial support can be added with:
[map] wms_groups = streets, buildings
[group_streets] wms_title = Streets wms_layers = streets_layer1, streets_layer1_labels wms_name = streets