The device is a generic method for accessing the underlying interface
with the native graphics subsystem, typically the X connection or
perhaps the GL context. By exposing a cairo_device_t on a surface and
its various methods we enable finer control over interoperability with
external interactions of the device by applications. The use case in
mind is, for example, a multi-threaded gstreamer which needs to serialise
its own direct access to the device along with Cairo's across many
threads.

Secondly, the cairo_device_t is a unifying API for the mismash of
backend specific methods for controlling creation of surfaces with
explicit devices and a convenient hook for debugging and introspection.

The principal components of the API are the memory management of:

  cairo_device_reference(),
  cairo_device_finish() and
  cairo_device_destroy();

along with a pair of routines for serialising interaction:

  cairo_device_acquire() and
  cairo_device_release()

and a method to flush any outstanding accesses:

  cairo_device_flush().

The device for a particular surface may be retrieved using:

  cairo_surface_get_device().

The device returned is owned by the surface.