Add support for index-only bbox queries

Index-only bbox queries can be evaluated very efficiently by spatial databases, but can contain false-positives. Still, for some case use-cases (e.g. rendering of features) they are preferable.

This patch tries to integrate them in a clean manner:

* Extends the BBOX-operator with a flag that indicates that false-positives are acceptable
* FeatureLayer sets this flag, so it is applied for WMS vector rendering
* If this flag is set, PostGISWhereBuilder maps the BBOX-operator to a faster, index-only BBOX check