Inverted polygons: bug fixes

* Fix feature count
* Avoid inplace geometry transformation (fix labeling issues)
* The geometry union has been reworked and now uses a "unary union" which is faster than the previous iterative approach (http://lin-ear-th-inking.blogspot.fr/2007/11/fast-polygon-merging-in-jts-using.html - as pointed out by @wonder-sk, thanks !). Unfortunately it was also less robust than before: the whole union was canceled when one of the geometry was invalid. So, now, a buffer(0) is forced if one of the geometry is invalid. It is then faster and more robust than the previous merging approach, but still slower than the default rendering method of course (without union)