Add limited <canvas> support

[GoogleCodeExporter]

Feature request:
1. add canvas component that supports rendering image or svg.

To learn Go, I am porting a python utility based on matplotlib to Go. 
Matplotlib provides browser-based rendering backend using a canvas element.  
http://matplotlib.org/users/whats_new.html#backends And, I was attracted to 
Gowut as a way to create the UI with Go.

The issue I encountered with reloading the current gowut image component within 
a gowut page is that there was obvious blinking as the image was removed and 
reloaded.  Drawing to a canvas element could avoid this.

Would adding minimal support for a canvas component to Gowut that allows 
drawing images be welcome?

If you do not have time to make this change, but think it would be a good idea, 
can you recommend how to approach it within the Gowut framework?  I'd be happy 
to give it a "go".

Thank you.

Original issue reported on code.google.com by [email protected] on 6 Mar 2014 at 5:04