Change --lossy algorithm to use current colorspace.

* The loss score is scaled to errors in the current colorspace by
  normalizing to the distance between rgb(0,0,0) and rgb(0,0,LOSS).
* Reset accumulated error at the start of each line (don't want to
  carry error into a different region of the image).
* The previous difference algorithm would accumulate differences
  in the difference score. The new algorithm does not. Rather, the
  error term is used only to eliminate candidates. A color is a
  valid candidate iff it is both close enough to the real pixel,
  and close enough to the real pixel modified by accumulated error.
* Rather than choosing the longest sequence with minimum error,
  compute a score that considers both length and error.

The results look reasonable, but the argument to `--lossy` has
different effects. For `--gamma=1`, this commit results
in better compression than the former commit (1.644M v. 2.202M),
on the falling-beecat gif from pull request #16,
at the cost of modest posterization (long runs of the same pixel,
rather than dithering). For `--gamma=srgb`, this commit
compresses worse than the old version, but with fewer artifacts,
and `--lossy=40` still looks ok.

src/gifsicle.c

@@ -899,18 +899,14 @@ set_new_fixed_colormap(const char *name)
 static void
 do_colormap_change(Gif_Stream *gfs)
 {
-  if (active_output_data.colormap_fixed || active_output_data.colormap_size > 0)
-    kc_set_gamma(active_output_data.colormap_gamma_type,
-                 active_output_data.colormap_gamma);
-
   if (active_output_data.colormap_fixed)
     colormap_stream(gfs, active_output_data.colormap_fixed,
                     &active_output_data);
 
   if (active_output_data.colormap_size > 0) {
     kchist kch;
     Gif_Colormap* (*adapt_func)(kchist*, Gt_OutputData*);
-    Gif_Colormap *new_cm;
+    Gif_Colormap* new_cm;
 
     /* set up the histogram */
     {
@@ -1025,6 +1021,8 @@ merge_and_write_frames(const char *outfile, int f1, int f2)
       w = active_output_data.resize_width;
       h = active_output_data.resize_height;
     }
+    kc_set_gamma(active_output_data.colormap_gamma_type,
+                 active_output_data.colormap_gamma);
     if (active_output_data.scaling != GT_SCALING_NONE)
       resize_stream(out, w, h, active_output_data.resize_flags,
                     active_output_data.scale_method,

src/gifsicle.h

@@ -280,6 +280,12 @@ enum {
 int     set_dither_type(Gt_OutputData* od, const char* name);
 void    colormap_stream(Gif_Stream*, Gif_Colormap*, Gt_OutputData*);
 
+struct kchist;
+Gif_Colormap* colormap_blend_diversity(struct kchist* kch, Gt_OutputData* od);
+Gif_Colormap* colormap_flat_diversity(struct kchist* kch, Gt_OutputData* od);
+Gif_Colormap* colormap_median_cut(struct kchist* kch, Gt_OutputData* od);
+
+
 /*****
  * parsing stuff
  **/