treeview: minor bugfix in the accuracy of selecting a node with the mouse.
treeview: minor bugfix in the accuracy of selecting a node with the mouse.

An earlier commit fixed the selected rectangle offset painting.
I forgot to also fix the node selection via mouse click. They y
offsets was a bit off.

wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI