Fix for problem saving from chaco shell.  Backbuffering was interfering with rendering off-screen.
Fix for problem saving from chaco shell.  Backbuffering was interfering with
rendering off-screen.

tommy-u/chaco,burnpanck/chaco,ContinuumIO/chaco,ContinuumIO/chaco,ContinuumIO/chaco,burnpanck/chaco,burnpanck/chaco,tommy-u/chaco,ContinuumIO/chaco,tommy-u/chaco