EMF Image size support

[andymac-2]

ref #3798

Correct aspect ratio for *.emf files when exporting to docx. There are two issues however unrelated to this PR:

1. If using multiple of the same image in a docx, the docx will resize every image (not just .emf) to the size of the first image. The size descriptions of the other copies are ignored. (pandoc does not use different sizes if same image is included several times (md -> docx) #3930)
2. .emf files exported by Inkscape are non-compliant to the specification of emf, so don't use these images for testing.

The only emf files I could find that weren't generated by Inkscape were included by default on windows.

[jgm]

Terrific, thanks!