Enable rel=features to return scale-appropriate features

[prushforth]

This requirement can be summarized as follows: Ensure that a layer requested via <map-link type=text/mapml rel=features tref="...request=GetMap&format_options=mapmlfeatures:true;..."> returns the feature that matches the calculated scale of the request, determined from the bbox, width and height parameters, regardless of whether that layer is an independent layer stored according to the pregeneralized-features extension OR the layer is a member of a layer group "style group" with the scale-dependency of internal layers defined by SDL min/max scale denominators. The latter scenario should be enabled via the other issues in this milestone

I tested the MapML module with the pregeneralized features module. MapML features are returned at the highest resolution of a pregeneralized layer, apparently without taking the scale of the request into account. (Maybe I did something wrong - it's the first time I've used the pregen module).

Here is a screen cap of a WMS image layer having a pregeneralized-features data source, at the smallest scale, with "Use Features" unchecked:

Here is the same layer served as features ("Use Features" checked):

Although it may be hard to discern on casual inspection, the second request depicted took about 1 minute to process, and there are a lot of details present that are not necessary at this scale e.g. many small islands in Hudson's Bay. The browser tab is consuming about 2/3 Gb of RAM with the one layer alone.

[aaime]

Some additional notes:

• Generated geometries must be topologically valid
• It's possible to drop small geometries that are not visible or covered by other geometries (e.g., ScreenMap approach in GeoTools).

Also going to look into extra techniques to reduce the size of output further (more generalization, more aggressive ones).