Oops we were accumulating paths during each spiral iteration and so the
tests were getting slower and slower and slower...
[And fix a couple of other instances of path accumulation.]
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.
The map for this test case was originally demonstrated as a
performance problem in this mozilla bug report:
A very slow SVG file with <path>s
https://bugzilla.mozilla.org/show_bug.cgi?id=332413
I obtained permission from the creator of the original file to
include the data here, (see comments in world-map.h for details).
