Function push_rectangle_indices 

pub fn push_rectangle_indices(
    ul: u32,
    ur: u32,
    dl: u32,
    dr: u32,
    out: &mut Vec<[u32; 3]>,
)

Given four corner points, pushes to two counterclockwise triangles to out.

Arguments:

• ul - the up-left point.
• dl - the down-left point.
• dr - the down-right point.
• ur - the up-right point.