New terminal driver: set term canvas

[Ethan M. <merritt@u.washington.edu>]

I have added Bruce Lueckenhoff's great new terminal driver for the
HTML 5 canvas to CVS.  This is nice work (thanks, Bruce!).  

Check out the full demo set as rendered by "set term canvas":

   http://skuld.bmsc.washington.edu/~merritt/gnuplot/canvas_demos/

You need a browser that supports HTML 5, which as I understand it
means firefox, opera, safari, or IE (I think IE needs a plugin?).

The new terminal supports all of the new gnuplot graphics options
including alpha-channel transparency, and as shown on the front
page of the demo set offers promise for support of client-side mousing.

The only real weak points are the lack of universal browser support
for HTML 5 and the poor-to-nonexistent support for text output in
the canvas spec.  The driver is currently using a public domain
javascript implementation of a basic ascii font from the Hershey
font set.  The Hershey collection also contains greek fonts, etc,
so if no better option comes along we could port more characters
to the javascript implementation for scientific plotting.


People working on SVG please take note:
---------------------------------------
There is now a new directory,  .../term/js, in the source tree.
It is installed in /usr/local/share/gnuplot/4.3/ in parallel to
the .../term/PostScript directory, and selected by the analogous
environmental variable GNUPLOT_JS_DIR. 

The "interactive SVG" patches that are floating around can 
share this directory to hold their javascript files.
 

-- 
Ethan A Merritt