fn box_blur_impl(
blur_radius_horz: usize,
blur_radius_vert: usize,
backbuf: &mut ImageRefMut<'_>,
frontbuf: &mut ImageRefMut<'_>,
)fn box_blur_impl(
blur_radius_horz: usize,
blur_radius_vert: usize,
backbuf: &mut ImageRefMut<'_>,
frontbuf: &mut ImageRefMut<'_>,
)