The mi filled arc code estimates that a filled arc will produce no
more spans than the arc is tall. This is true for most arcs except
for pie-slice arcs strictly between 180 and 360 degrees where the missing
portion of the arc faces up or down such that we get two spans on some
scanlines.

For those, we need to reserve room for another height/2 spans. This
patch just does it for all partial pie-sliced arcs to make the test
easier to understand; it's just over-allocating a bit of memory, so
that's safe.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>