djpeg -crop: Exit gracefully with non-PPM formats

... 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

djpeg.1

@@ -1,4 +1,4 @@
-.TH DJPEG 1 "18 March 2017"
+.TH DJPEG 1 "8 November 2017"
 .SH NAME
 djpeg \- decompress a JPEG file to an image file
 .SH SYNOPSIS
@@ -204,7 +204,8 @@ Decompress only a rectangular subregion of the image, starting at point X,Y
 with width W and height H.  If necessary, X will be shifted left to the nearest
 iMCU boundary, and the width will be increased accordingly.  Note that if
 decompression scaling is being used, then X, Y, W, and H are relative to the
-scaled image dimensions.
+scaled image dimensions.  Currently this option only works with the
+PBMPLUS (PPM/PGM) output format.
 .TP
 .B \-verbose
 Enable debug printout.  More