image/jpeg: fix progressive decoding when the DC components are split

image/jpeg: fix progressive decoding when the DC components are split
over multiple scans. Previously, the Go code assumed that DC was
synonymous with interleaved and AC with non-interleaved.

Fixes #6767.

The test files were generated with libjpeg's cjpeg program, version 9a,
with the following patch, since cjpeg is hard-coded to output
interleaved DC.

$ diff -u jpeg-9a*/jcparam.c
--- jpeg-9a-clean/jcparam.c	2013-07-01 21:13:28.000000000 +1000
+++ jpeg-9a/jcparam.c	2014-02-27 11:40:41.236889852 +1100
@@ -572,7 +572,7 @@
 {
   int ci;

-  if (ncomps <= MAX_COMPS_IN_SCAN) {
+  if (0) {
         /* Single interleaved DC scan */
         scanptr->comps_in_scan = ncomps;
         for (ci = 0; ci < ncomps; ci++)
@@ -610,7 +610,7 @@
           (cinfo->jpeg_color_space == JCS_YCbCr ||
                cinfo->jpeg_color_space == JCS_BG_YCC)) {
         /* Custom script for YCC color images. */
-    nscans = 10;
+    nscans = 14;
   } else {
         /* All-purpose script for other color spaces. */
         if (ncomps > MAX_COMPS_IN_SCAN)

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/69000046