[Xcb] magical callback funness

[Jamey Sharp]

AIEEEEE!

>From the RECORD protocol spec: "[I]ntercepted protocol data are returned
to recording clients through multiple replies to the extension request
to begin protocol interception and reporting.  This utilization is
consistent with ListFontsWithInfo, for example, where a single request
has multiple replies."

*sob* I *hate* ListFontsWithInfo...

OK, apparently we need some general mechanism for XCB to handle multiple
replies with the same sequence number. Since I refuse to add callback
hooks to XCB again, and since I suppose clients will want each reply
with minimum delay (so no waiting for all replies and returning an
array), I guess XCB will need to store a queue of replies, and return
one per call to XCBWaitForReply. 

We can add a tag to the XML-XCB bits, whose only effect in XCB is to set
a new flag in the XCBProtocolRequest structure. Josh, I'll be relying on
you to implement that: do you have any comments?

XCB can automatically tell that the last reply has arrived, when a
response of any sort having a higher sequence number arrives. Finding an
opportunity to remove the reply record is a little tricky, so I suppose
a first implementation will leak a few bytes for each of these evil
requests sent and we'll fix it once we've got the basics in place.

Xlib/XCB's implementation of multiple-reply handling will need a little
bit of thought: my current support for async-handlers would, I think,
cause Xlib to drop half the replies, skipping every other one. It should
be fixable, but I'll just have to think about it.

What a pain.

So XCB won't have a functioning RECORD extension until we get this stuff
straightened out. I hope nobody was in urgent need of it...

--Jamey