Further partial image decompression fixes

- Referring to 073b0e88a1 and #185, the
  reason why BMP and RLE didn't (and won't) work with partial image
  decompression is that the output engines for both formats maintain a
  whole-image buffer, which is used to reverse the order of scanlines.
  However, it was straightforward to add -crop support for GIF and
  Targa, which is useful for testing partial image decompression along
  with color quantization.
- Such testing reproduced a bug reported by Mozilla (refer to PR #182)
  whereby jpeg_skip_scanlines() would segfault if color quantization was
  enabled.  To fix this issue, read_and_discard_scanlines() now sets up
  a dummy quantize function in the same manner that it sets up a dummy
  color conversion function.

Closes #182

djpeg.c

@@ -31,7 +31,6 @@
 #include "cdjpeg.h"             /* Common decls for cjpeg/djpeg applications */
 #include "jversion.h"           /* for version message */
 #include "jconfigint.h"
-#include "wrppm.h"
 
 #include <ctype.h>              /* to declare isprint() */
 
@@ -173,7 +172,7 @@ usage (void)
 
   fprintf(stderr, "  -skip Y0,Y1    Decompress all rows except those between Y0 and Y1 (inclusive)\n");
   fprintf(stderr, "  -crop WxH+X+Y  Decompress only a rectangular subregion of the image\n");
-  fprintf(stderr, "                 [requires PBMPLUS (PPM/PGM) output format]\n");
+  fprintf(stderr, "                 [requires PBMPLUS (PPM/PGM), GIF, or Targa output format]\n");
   fprintf(stderr, "  -verbose  or  -debug   Emit debug output\n");
   fprintf(stderr, "  -version       Print version information and exit\n");
   exit(EXIT_FAILURE);
@@ -714,11 +713,10 @@ main (int argc, char **argv)
     }
 
     jpeg_crop_scanline(&cinfo, &crop_x, &crop_width);
-    if (requested_fmt != FMT_PPM)
+    if (dest_mgr->calc_buffer_dimensions)
+      (*dest_mgr->calc_buffer_dimensions) (&cinfo, dest_mgr);
+    else
       ERREXIT(&cinfo, JERR_UNSUPPORTED_FORMAT);
-    ((ppm_dest_ptr) dest_mgr)->buffer_width = cinfo.output_width *
-                                              cinfo.out_color_components *
-                                              sizeof(JSAMPLE);
 
     /* Write output file header.  This is a hack to ensure that the destination
      * manager creates an output image of the proper size.