[Xcb] Xlibs pretty print

[Jeremy Kolb]

If you are running normal xlibs and you run into a request/reply that 
the server does not support you get a pretty little error message that 
tells you so.  If you are running xlibs/xcb you get something like:

arbfptest1: xcl/io.c:276: _XReply: Assertion `!dpy->xcl->reply_data' failed.

I haven't looked at it yet but is it possible to use xlib's function for 
this or do the xcb hooks make that not likely?  I remember on the wiki 
we have something about pretty printing but that hasn't been updated in 
ages.

Jeremy