Returns the difference between two geometries.
The result will be a geometry consisting of all the parts of the first geometry that are not in the second geometry. Thus, the most complex geometry must be passed in as parameter 1.
Parameter 1 - any geometry.
Parameter 2 - another geometry (to be subtracted from the first geometry).
Returns: a geometry equal to geometry1 AND NOT geometry2, or null if either parameter is not a geometry.
Note: This function does not currently fully support 3D geometries. Any 3D geometries will be projected down to 2 dimensions.