[cairo] Analytical curve bounds

[Behdad Esfahbod]

Kalle Vahlman wrote:

> Attached is the output of 'cairo-perf-diff ef0f6c^ ef0f6c' (only the
> image backend) on Acer Aspire One. There's loads of fill tests that
> are faster, but also few noticeable slowdowns.

Thanks Kalle.  Since all the results are in thee 20% difference range I wonder
how much of it is noise.  In fact, I wonder how much of the tests actually hit
the new code path at all.  I think we need a test that does a lot of
cairo_fill_extents() in a loop to exercise the new code.  Even that I'm not
sure.  Chris knows which goes through which path...

behdad