A lockup was reported by fargoilas on #clutter and removing the server grab in
clutter_x11_texture_pixmap_sync_window fixed the problem.
We were doing an x server grab to guarantee that if the XGetWindowAttributes
request reported that our redirected window was viewable then we would also
be able to get a valid pixmap name. (the composite protocol says it's an
error to request a name for a window if it's not viewable) Without the grab
there would be a race condition.
Instead we now handle error conditions gracefully.
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