Inverted polygons: fix memory leaks

Fix memory leaks in the inverted polygon symbology
It also fixes a small one in the GUI part (single symbol renderer widget).
Tested with google perftools heap leak checker (I should have done that in the first place :( )
 
What about compatibility with GEOS < 3.3 that does not have unary union? The code would not compile there... (That was the reason for using _makeUnion())