[Xcb] 0 sequence number for request errors?

[Barton C Massey]

One of the biggest holes in current XCB API is that there's
no way to find out that a request failed to be sent---in
particular, a request that doesn't require a reply.

One hack to get this behavior would be to "reserve" the
sequence number 0 for an error return.  IOW, a request that
returned a sequence number of 0 has failed.  This would
require skipping sequence number 0 by issuing a noop inside
XCB at this point.

Obviously, this isn't the prettiest solution.  But it could
work, and would solve a real problem.  Comments?  Alternate
suggestions?

    Bart