removed rounding of values in composeSMaskLuminosity

maskData comes out of maskCtx.getImageData, so is 0..255 clamped, and
the used multiplications will not create fractions needing rounding,
neither would addition.

Not a big deal, but composeSMaskLuminosity pops up in profiles, so at
least its somewhat relevant, even when the operator itself disappears in
noise when trying to profile it with the limited tools we have in js.