Fix Xlib/XCB for multi-threaded applications (with caveats).

Rather than trying to group all response processing in one monolithic
process_responses function, let _XEventsQueued, _XReadEvents, and
_XReply each do their own thing with a minimum of code that can all be
reasoned about independently.

Tested with `ico -threads 20`, which seems to be able to make many
icosahedrons dance at once quite nicely now.

Caveats:

- Anything that was not thread-safe in Xlib before XCB probably still
  isn't. XListFontsWithInfo, for instance.

- If one thread is waiting for events and another thread tries to read a
  reply, both will hang until an event arrives. Previously, if this
  happened it might work sometimes, but otherwise would trigger either
  an assertion failure or a permanent hang.

- Versions of libxcb up to and including 1.6 have a bug that can cause
  xcb_wait_for_event or xcb_wait_for_reply to hang if they run
  concurrently with xcb_writev or other writers. So you'll want that fix
  as well.

Signed-off-by: Jamey Sharp <jamey@minilop.net>
Reviewed-by: Josh Triplett <josh@freedesktop.org>

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