add read-only UI contentOffset property

src/graphics/ui/elements/UINode.cpp

@@ -152,7 +152,7 @@ float UINode::getTooltipDelay() const {
 
 glm::vec2 UINode::calcPos() const {
     if (parent) {
-        return pos + parent->calcPos() + parent->contentOffset();
+        return pos + parent->calcPos() + parent->getContentOffset();
     }
     return pos;
 }