Function assert_draws_its_trims 

pub fn assert_draws_its_trims(part: &NamedModel)

Checks every face of part is drawn exactly where its trims say: the triangles the rasterizer draws it with cover, in (u, v), the area its outer boundary encloses less its holes’ — as the rasterizer samples them. A face drawn over a hole it goes around, or leaving a gap, covers more or less. Plain f64, like the rasterizer.