Working shared whiteboard! (kind-of)

[David Malcolm <dmalcolm redhat com>]

I just committed my latest whiteboard code to CVS.  It's also something
of a blackboard for now; screenshot available here:
http://people.redhat.com/dmalcolm/yarrr/Whiteboard-Screenshot.png

Strokes start out client-side in red; they are drawn server-side in
green.  The server-side image contains the version number, which is
handy for debugging.

Unfortunately, I'm beset by coordinate-space issues; there doesn't seem
to be a good cross-browser way of getting from mouse event coordinates
to image coordinates for the whiteboard canvas.  So, as an evil kludge,
I've moved the whiteboard to the top left of the page.

So it works (ish); we have shared whiteboards, only needing javascript
on the clientside.  

There's a lag where it removes a client-side stroke whilst downloading
the new server-side image containing that stroke, so strokes temporarly
vanish during this download.  Ideally we'd listen for "image reload
complete" events; dunno how to do this in a cross-browser way.

Dave