[cairo] Tessellator performance patch

[chris nuernberger]

But in both of those cases you ascii'd out, you know that the sign of
ax-bx changes if you take different end points of the segments.
Really, you only need the sign of a subtracting at the oppose end from
the point you are at from the intersection point, and you would know
if a collision is possible.  This means that if you know the
intersection point should lie below you, then you check the sign of
the subtraction of the end x points at the lower y.  Or reverse this
logic.

Chris