JPEG XR import/export support

Live previews are having some trouble, but as far as I can tell it's not
anything to do with my code.  I'm not too worried about it, given the
rarity of the format and the fact that FreeImage is still in the midst
of a dev cycle, but if it doesn't go away by the time 6.2 releases, I'll
revisit.

(The actual saving to file works fine.)