Add support for clients to request the server insert breakpoints at
various points in its processing. These breakpoints are handled
internally by semaphores (visible to tests through helpers): when the
server reaches the specified point, it will pause execution until the
client allows it to restart.

A weston_compositor pointer returned at each breakpoint allows the
client to reach across the thread boundary and access the server's
internal data structures. This can be used to, for example, inspect
paint nodes, internal damage, or any other work which is not necessarily
client-visible.

The majority of tests will not need to use this infrastructure; it is
only intended for tightly-coupled tests which can very specifically
dictate and anticipate the server's execution flow.

Signed-off-by: Daniel Stone <daniels@collabora.com>