The 3-D graphics in Towers need a corresponding change on the input

The 3-D graphics in Towers need a corresponding change on the input
side: instead of locating a mouse click in whichever base grid
square contained the click location, we should check to see if a
tower protrudes on to that location from a neighbouring square. That
way, a click should reliably activate the tower that was actually
drawn under the mouse pointer.

[originally from svn r8826]