image scaling

[Stefan Monnier <monnier@iro.umontreal.ca>]

Looking at the Elisp manual's "38.16.2 Image Descriptors", it seems that
we do not support image scaling.

I'd like to add such a feature to image-mode.el, but am wondering how we
should go about doing it.  Would there be some advantage to provide it
at the C level (e.g. if some of the image libs we use already provide
the necessary functionality) or should we just bite the bullet and call
some external tool like "convert -scale ..." (with the disadvantage that
flushing the image cache will not automatically cause the image to be
re-scaled).


        Stefan