Make the operator strength reduction constant time.

Make the operator strength reduction constant time.

By extending the operator information table to cover all operators we
can replace the loop with a table look-up. At the same time, base the
operator optimization on the computed flags rather than the ones in
the image struct.

Finally, as an extra optimization, we no longer ignore the case where
there is a mask. Instead we consider the source opaque if both source
and mask are opaque, or if the source is opaque and the mask is
missing.