[cairo] Condition variables

[Chris Wilson]

I've extended the cairo-test framework to run tests across multiple
threads in an effort to stress cairo and uncover races. So far I've hit
two from cairo-xlib-surface. The first was a fairly trivial case where
the display list could be updated whilst
_cairo_xlib_call_close_display_hooks() was running. The second is a
little more difficult as it involves a surface being removed from the
solid-surface-cache and thus destroyed, whilst simultaneously it is
being detached from the display due to a XCloseDisplay. The only sane
way I can see of avoiding this race is by stalling the
_cairo_xlib_surface_finish() whilst the display is being closed (after
which the resources are marked as freed and so no work is queued from
the _finish()). So in order to stall the function I require a condition
variable. I've attached a patch for the pthread case, but I don't know
how trivial condvars are on other architectures (e.g. GLib's
implementation for win32 is definitely not trivial!).
--
Chris Wilson
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