Fix for Bug #6942

While this bug is not going to break the program if it isn't fixed, if anyone wanted to add a legend with a font which has a descent > 0, the bounding box was not calculated correctly which led to letters like g and j to be cut off below the base line. I disagree strongly that a bug like this isn't really important to fix because most users will create a map and all good maps have a legend and if a letter is cut off in the legend title, it looks really unprofessional. 

... are not cut off from the legend title.