add textbox:indexByPos and :lineY methods

src/graphics/ui/elements/UINode.hpp

@@ -267,7 +267,7 @@ namespace gui {
         virtual glm::vec4 calcColor() const;
 
         /// @brief Get inner content offset. Used for scroll
-        virtual glm::vec2 getContentOffset() {return glm::vec2(0.0f);};
+        virtual glm::vec2 getContentOffset() const {return glm::vec2(0.0f);};
         /// @brief Calculate screen position of the element
         virtual glm::vec2 calcPos() const;
         virtual void setPos(const glm::vec2& pos);