Add GDALVector class methods setSpatialFilter() / getSpatialFilter()

[ctoney]

$setSpatialFilter(wkt)

' Sets a new spatial filter from a geometry in WKT format. This method sets

' the geometry to be used as a spatial filter when fetching features via the

' $getNextFeature() or $fetch() methods. Only features that geometrically

' intersect the filter geometry will be returned. Currently this test may be

' inaccurately implemented (depending on the vector format driver), but it is

' guaranteed that all features whose envelope overlaps the envelope of the

' spatial filter will be returned. This can result in more shapes being

' returned that should strictly be the case.

' wkt is a character string containing a WKT geometry in the same coordinate

' system as the layer. An empty string ("") may be passed indicating that

' the current spatial filter should be cleared, but no new one instituted.

$getSpatialFilter()

' Returns the current spatial filter geometry as a WKT string, or empty string

' ("") if a spatial filter is not set.