Fix fallback canvas rendering

[bwlewis]

Canvas and WebGL rendering code has been diverging for some time, and finally now in 0.3.0 Canvas is no longer working. The big difference is the extensive use of buffering for performance on the WebGL side. All of that code is completely different for Canvas.

I think most people are using this for the fast WebGL graphics, so maybe this is not a big deal and I'm marking this "wontfix" for now, but leave open the possibility of re-visiting this and fixing it.

I have temporarily disabled canvas in the package with an error message. If someone really wants to re-enable this and get it working, help is appreciated!