[Xcb] print backtraces in case an assert fails inside xlib/xcb

[Barton C Massey]

In message <5b9d31e90706071146t15797b3fi66bbea25bced2ac9 at mail.gmail.com> you wrote:
> I mean, will XCB team support users of patched XCB
> library?

Sure, to the extent we can!  We want everybody's stuff to
work...

But I don't *think* we're going to start shipping the patch
ourselves anytime soon, for the reasons given in my previous
message.

(Note that most binary-only X apps won't be affected by this
issue, since most of them are dynamically linked against X
libraries and themselves don't call any Xlib locking
functions.  The assertion failure is actually a rare problem
now that we have the X libraries mostly fixed and have Java
mostly fixed; users with those fixes seem generally happy
with the locking assertions turned on as far as I know.)

    Bart