Remove Pixels: Mul<Pixels, Output = Pixels> impl, add ScaledPixels ops

[mgsloan]

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