Better DC pass (#1006)

codecs/mozjpeg/enc/mozjpeg_enc.cpp

@@ -141,6 +141,7 @@ val encode(std::string image_in, int image_width, int image_height, MozJpegOptio
   jpeg_c_set_bool_param(&cinfo, JBOOLEAN_TRELLIS_EOB_OPT, opts.trellis_opt_zero);
   jpeg_c_set_bool_param(&cinfo, JBOOLEAN_TRELLIS_Q_OPT, opts.trellis_opt_table);
   jpeg_c_set_int_param(&cinfo, JINT_TRELLIS_NUM_LOOPS, opts.trellis_loops);
+  jpeg_c_set_int_param(&cinfo, JINT_DC_SCAN_OPT_MODE, 0);
 
   // A little hacky to build a string for this, but it means we can use
   // set_quality_ratings which does some useful heuristic stuff.