add a few simple helpers to Image

[cyrgani]

This PR adds the public helper method pixel_amount, which is simpler than always having to write image.width as usize * image.height as usize.
It also adds the private helper assert_same_size to avoid repeating the following larger assertion several times:

assert!(
    self.width as usize * self.height as usize
        == other.width as usize * other.height as usize
);

Both helpers are used in the other Image methods when possible.
Part of #748.