[Xcb] XCB Errors and Handling

[Ori Bernstein]

Hi. I'm currently trying to port Xnest to XCB, and am getting rather strange
errors when running it.

XCBPollForEvent() returns an event with response_type == 0, but with a
(seemingly) valid sequence number. The error returned in the error argument to
XCBPollForEvent is 0, so I'm not sure what's going wrong. 

The actual code that's doing this can be viewed here, although the debugging
prints haven't been added.:

http://gitweb.freedesktop.org/?p=xorg-xserver;a=blob;h=9c30a18340838a4c2c50d58600f824520cf642c6;hb=c15aa3a0cf772622e3271015dc53f6822cd9dce9;f=hw/xnest/Events.c,

There is some other general brokenness in the code, but right now I'm trying to
fix the event problems, since I'm hoping it's causing some of the other
breakage.