In order to handle 'cairo-perf-trace benchmark', we need to perform the
can_run? test on the directory name as opposed to the individual trace
names. Make it so.
By ensuring that tests take longer than a couple of seconds we eliminate
systematic errors in our measurements. However, we also effectively
eliminate the synchronisation overhead. To compensate, we attempt to
estimate the overhead by reporting the difference between a single
instance and the minimum averaged instance.
Add a new test case to Cairo for checking the performance of Cairo's
equivalent to GDK's gdk_pixbuf_composite_color() operation. That is an
operation that happens to be extremely useful when viewing or editing
transparent images so I think it is important that it is as fast as
possible.
