Support for opaque WMS layers

The WMS spec says about the opaque layer property : "If the optional Boolean attribute opaque is absent or false, then maps made from that Layer will generally have significant no-data areas that a client may display as transparent. Vector features such as points and lines are considered not to be opaque in this context (even though at some scales and symbol sizes a collection of features might fill the map area). A true value for opaque indicates that the Layer represents an area-filling coverage. For example, a map that represents topography and bathymetry as regions of differing colours will have no transparent areas. The opaque declaration should be taken as a hint to the Client to place such a Layer at the bottom of a stack of maps."