Add option to have single DC scan

Add option to have a single DC scan wherein all components are
interleaved when using progressive mode. This may resolve compatibility
issues raised in #29 and #48.
This option is available through -onedcscan in cjpeg

cjpeg.c

@@ -170,6 +170,7 @@ usage (void)
 #endif
   fprintf(stderr, "  -revert        Revert to standard defaults (instead of mozjpeg defaults)\n");
   fprintf(stderr, "  -fastcrush     Disable progressive scan optimization\n");
+  fprintf(stderr, "  -onedcscan     Use a single DC scan containing all components\n");
   fprintf(stderr, "  -notrellis     Disable trellis optimization\n");
   fprintf(stderr, "  -tune-psnr     Tune trellis optimization for PSNR\n");
   fprintf(stderr, "  -tune-hvs-psnr Tune trellis optimization for PSNR-HVS (default)\n");
@@ -342,6 +343,9 @@ parse_switches (j_compress_ptr cinfo, int argc, char **argv,
 	lval *= 1000L;
       cinfo->mem->max_memory_to_use = lval * 1000L;
 
+    } else if (keymatch(arg, "onedcscan", 3)) {
+      cinfo->one_dc_scan = TRUE;
+      
     } else if (keymatch(arg, "optimize", 1) || keymatch(arg, "optimise", 1)) {
       /* Enable entropy parm optimization. */
 #ifdef ENTROPY_OPT_SUPPORTED

jcmaster.c

@@ -754,7 +754,7 @@ select_scans (j_compress_ptr cinfo, int next_scan_number)
     
     copy_buffer(cinfo, 0);
 
-    if (cinfo->num_scans > cinfo->num_scans_luma) {
+    if (cinfo->num_scans > cinfo->num_scans_luma && !cinfo->one_dc_scan) {
       base_scan_idx = cinfo->num_scans_luma;
       
       if (master->interleave_chroma_dc)

jcparam.c

@@ -702,6 +702,9 @@ jpeg_search_progression (j_compress_ptr cinfo)
   /* last 4 done conditionally */
   
   /* luma DC by itself */
+  if (cinfo->one_dc_scan)
+    scanptr = fill_dc_scans(scanptr, ncomps, 0, 0);
+  else
     scanptr = fill_dc_scans(scanptr, 1, 0, 0);
   
   scanptr = fill_a_scan(scanptr, 0, 1, 8, 0, 0);
@@ -824,8 +827,12 @@ jpeg_simple_progression (j_compress_ptr cinfo)
     if (cinfo->use_moz_defaults == TRUE) {
       /* scan defined in jpeg_scan_rgb.txt in jpgcrush */
       /* Initial DC scan */
+      if (cinfo->one_dc_scan)
+        scanptr = fill_dc_scans(scanptr, ncomps, 0, 0);
+      else {
         scanptr = fill_dc_scans(scanptr, 1, 0, 0);
         scanptr = fill_a_scan_pair(scanptr, 1, 0, 0, 0, 0);
+      }
       /* Low frequency AC scans */
       scanptr = fill_a_scan(scanptr, 0, 1, 8, 0, 2);
       scanptr = fill_a_scan(scanptr, 1, 1, 8, 0, 0);

jpeglib.h

@@ -378,6 +378,7 @@ struct jpeg_compress_struct {
 
   boolean use_moz_defaults; /* TRUE=use Mozilla defaults */
   boolean optimize_scans; /* TRUE=optimize progressive coding scans */
+  boolean one_dc_scan; /* TRUE=use a single DC scan interleaving all components */
   boolean trellis_quant; /* TRUE=use trellis quantization */
   boolean trellis_eob_opt; /* TRUE=optimize for sequences of EOB */
   boolean use_flat_quant_tbl; /* TRUE=use flat quantization table */