Require pixman >= 0.40
Cairo 1.18.0 does not compile with pixman < 0.39.0 due to a missing #if guard. As pixman 0.40.0 was released over 3 years ago we should just bump the minimum pixman to 0.40.0 to ensure dithering is always available for image surfaces.
I've also cleaned out some old pixman version guards.