updates to cl-cairo2 and cl-2d

Following the discussion and the resulting poll, I updated cl-cairo2 and consequently cl-2d to use the special variable *context* as the default context. This does not affect the interface of cl-2d (except for user-defined functions that draw directly on the cairo context, of course), since in cl2d, the context is always saved in the frame and there is no "default" frame. Nevertheless, updating the cl-2d code was beneficial because I was able to see if the new style makes things easier. It appears that it simplifies the code quite a bit: I would like to thank everyone who argued for the change. Enjoy. Bug reports and contributed code are appreciated.