Explicit support of Geoserver CQL_FILTER parameter

Support of CQL_FILTER filters in wms layers has been introduced, to allow correct merging of layers having a filter.

The fix allows merging of layers in a single wms request, also if they have CQL_FILTER defined (some or all of them). The filter is correctly composed using a ';' when multiple layers are merged, and the INCLUDE filter for no filtered layers.