Fix render issue with combining chars intersecting with the base char.

Example: text("A\u030A", font = "Arial");

The fix relies on the winding order of the polygons reported by FreeType
to be correct, so the Clipper union operation keeps that information.

src/DrawingCallback.cc

@@ -43,6 +43,7 @@ DrawingCallback::~DrawingCallback()
 void DrawingCallback::start_glyph()
 {
 	this->polygon = new Polygon2d();
+	this->polygon->setSanitized(true);
 }
 
 void DrawingCallback::finish_glyph()