Function derive 

pub fn derive(
    camera: &Camera,
    xres: u32,
    yres: u32,
    hx: f32,
    hy: f32,
    hz: f32,
) -> CameraState

Derive the per-frame CameraState.

Mirrors voxlaptest’s setcamera(ipo, ist, ihe, ifo, dahx, dahy, dahz) (voxlap5.c:2246) bit-exactly, including the f64 → f32 narrowing at the call boundary.