It doesn't make sense to have `Pixels: Mul<Pixels, Output = Pixels>` as
the output should be `Pixels^2` (area), so these impls are removed. All
code where these impls were used are improved by instead multiplying by
`f32` or `usize`.

Also adds math op impls that are present for `Pixels` but absent for
`ScaledPixels`. Adds missing `Mul<Pixels> for usize` to both.

Release Notes:

- N/A