Creating A Cairo Context Seems Quite Quick

I just added methods to my Path and Path.Segment types in Qahirah
<https://github.com/ldo/qahirah> to compute extents and return
flattened versions. These create a temporary Cairo context so I can use
its append_path and copy_path_flat calls to do all the work. The
overhead of this doesn’t seem too bad.

As a baseline (on an already-created Cairo context), the Python sequence

    ctx.new_path().append_path(path).copy_path_flat()

takes 140µs for a simple rectangular path on my Core i7 (averaged over
100,000 iterations using the Python timeit module), while

    ctx.new_path().append_path(path).path_extents

takes 59µs. For the new methods I have added, which create a new context
on every call,

    path.flatten()

takes 162µs, while

    path.extents

takes 83µs. So the overhead in the former is about 16%, in the latter
about 40%. Well short of the order-of-magnitude-or-more I was
expecting/fearing...
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