... and document that only PPM/PGM output images are supported with the
-crop option for the moment.

I investigated the possibility of supporting -crop with -bmp, but even
after resetting the buffer dimensions, I still kept getting virtual
array access errors.  It seems that doing this the "right way" would
require creating a re-initialization function for each image format's
destination manager.  I'm disinclined to do that right now, given that
this feature was Google's baby (developed as a prerequisite for
including libjpeg-turbo in Android), and the -crop option in djpeg is
intended only as an example of how to use the partial image
decompression API.  Real-world applications would need to handle this
in their own destination managers.

It would probably be possible to make this work with Targa by employing
a similar hack to the one we used with PPM, but Targa isn't popular
enough to bother.

Fixes #185