Addition of a thin-lens camera...

[Kevin Harris]

I have just added a thin-lens camera to panorama.  There may be bugs, and
I know it doesn't perfectly model a real camera (it is, afterall, an
approximation), but it works for my purposes.

An example image, generated from scenes/misc/blur.rt (at aa_depth=128) can
be found at
http://www.geocities.com/barthol_o_mule/graphical/images/blur.jpg

I hope someone will try to make some nice scenes with this, as it has the
capability to add a whole lot of realism.

Let me know what you think, or if you run into any problems with what
I've checked in.

btw. Is anyone else still planning on doing any work on panorama?

Also, I think we need to put up a screenshots (generated images page) on
sourceforge.  It would attract a lot more people to use (and possibly add
stuff to) panorama.  If Angel could help there (as nobody else can do it), 
I'd appreciate it. :)

-Kevin-