Bug 218187

Web Inspector WebGL canvas debugger is:
* reading uninitialised drawing buffers intentionally 
* displaying at best 3 buffers old content (at worst, seee above)
* is most likely corrupting the toDataURL / snapshot / printing cache bitmap

The problem the inspector hack is trying to solve is:
* need to capture snapshot of synchronously
* need to capture drawing buffer if drawing buffer has been drawn to
* need to capture display buffer if drawing buffer has not been draw to


Fix by:
* introducing a better function to read the canvas content
* implement display buffer read
* not pollute the HTMLCanvasElement "rendering results" image
* implement cairo toDataURL or route data through ImageBuffer