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]
3D effect. (I only committed it without so that I could preserve a
record of how the code looked before I added it.) The 3D effect can
be disabled by setting TOWERS_2D in the environment, should anybody
seriously dislike it.
[originally from svn r8817]