Fix for stroke transparency issue

(when an opque stroke was in the vicinity of a translucent one, it
became translucent itself)

src/domain/UBGraphicsPolygonItem.cpp

@@ -208,7 +208,7 @@ void UBGraphicsPolygonItem::copyItemParameters(UBItem *copy) const
 void UBGraphicsPolygonItem::paint ( QPainter * painter, const QStyleOptionGraphicsItem * option, QWidget * widget)
 {
     if(mHasAlpha && scene() && scene()->isLightBackground())
-        painter->setCompositionMode(QPainter::CompositionMode_Darken);
+        painter->setCompositionMode(QPainter::CompositionMode_SourceOver);
 
     painter->setRenderHints(QPainter::Antialiasing);