Formalize WMF/EMF support

I've worked out the last few items that were bothering me (such as PD
reporting different metafile sizes from MS Paint and Irfanview), so I'm
content to make metafile support official.

By default, antialiasing and high-quality interpolation of raster data
is enabled.  Ideally, an import dialog would be helpful, to not only let
the user set an arbitrary size in advance (while taking advantage of
lossless resizing), but to also let them toggle things like AA.  I've
made a note to investigate such a feature in the future.