Review cairo-test-trace.c and rewrite parts to ease understanding and fix
various bugs - such as failure to notice the slaves crashing and not
releasing our shared memory after an interrupt.
The basic premise is that we feed the trace to multiple backends in
parallel and compare the output at the end of each context (based on
the premise that contexts demarcate expose events, or their logical
equivalents) with that of the image[1] backend. Each backend is
executed in a separate process, for robustness, with the image data
residing in shared memory and synchronising over a socket.
[1] Should be reference implementation, currently the image backend is
considered to be the reference for all other backends.
