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 the performance test case to compare the speed of filling a rounded
rectangle (one with camphered corners) as opposed to an ordinary
rectangle. Since the majority of the pixels are identical, ideally the two
cases would take similar times (modulo the additional overhead in the more
complex path).
