More sensible color behavior when masking.

The output now ranges from the specified neutral color (where
there is no data) to the background color/transparency (where
there is enough for full saturation).

Example: -m -t 128 -w -c FF0000 from red to half-opaque white
         -m -t 0 -c 000000 from black to fully transparent

It never passes through into the overexposed range because I
don't know what it means to be more transparent than transparent.