support trailing semicolons for `gray_image!`

[cospectrum]

fn main() {
    let img = gray_image!(type: f32,
        1., 2., 3.;
        4., 5., 6.;
        7., 8., 9.;  // <- this one ";" is not supported
    );
 }

[theotherphil]

This would be a welcome change. I can’t remember why I didn’t support this initially - I suspect laziness!