Merged in the scale branch:

Merged in the scale branch:

- adds a new scaling mechanism 'mixing' based on the method
implemented by pnmscale.  Produces better detail when scaling down and
is faster than the 'normal' method.

- the scale() method can now scale non-proportionally if the caller
specifically asks for it with xscalefactor/yscalefactor or by setting
type to 'nonprop'.