cairo_get_target() returns the original surface passed to
cairo_create(), and not the current destination as required when
testing drawing to the same surface using multiple contexts.
For completeness we also use the group target when creating similar
surfaces within the tests (to check that similar surfaces of similar
surfaces also work).
This will finally allow us to very easily add lots of other
tests that will similarly involve iterating over the various
sources and operators of interest.
